Magnetic recording medium having characterized magnetic layer and magnetic recording and reproducing device

ABSTRACT

Provided are a magnetic recording medium, in which a magnetic layer includes ferromagnetic hexagonal ferrite powder, a binding agent, and an oxide abrasive, an intensity ratio Int(110)/Int(114) obtained by an X-ray diffraction analysis of the magnetic layer by using an In-Plane method is 0.5 to 4.0, a vertical squareness ratio of the magnetic recording medium is 0.65 to 1.00, a coefficient of friction measured regarding a base portion of a surface of the magnetic layer is equal to or smaller than 0.30, and an average particle diameter of the oxide abrasive obtained from a secondary ion image obtained by irradiating the surface of the magnetic layer with a focused ion beam is 0.04 μm to 0.08 μm, and a magnetic recording and reproducing device including this magnetic recording medium.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 U.S.C 119 to Japanese PatentApplication No. 2017-191665 filed on Sep. 29, 2017 and Japanese PatentApplication No. 2018-170192 filed on Sep. 12, 2018. Each of the aboveapplications is hereby expressly incorporated by reference, in itsentirety, into the present application.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to a magnetic recording medium and amagnetic recording and reproducing device.

2. Description of the Related Art

The recording and/or reproducing of information with respect to amagnetic recording medium is generally performed by bringing a surfaceof a magnetic recording medium (surface of magnetic layer) into contactwith a magnetic head (hereinafter, also referred to as a “head”) andsliding.

One performance required from the magnetic recording medium is toexhibit excellent electromagnetic conversion characteristics in a caseof reproducing information recorded on the magnetic recording medium.

Meanwhile, in a case where chipping of a reproducing element of the headoccurs due to the sliding between the surface of the magnetic layer andthe head (hereinafter, also referred to as “head element chipping”), adistance between the surface of the magnetic layer and the reproducingelement increases and spacing loss which is a reason of a deteriorationof electromagnetic conversion characteristics may occur. As thecountermeasure for preventing the occurrence of this spacing loss, atechnology of providing a protective layer on the head has been proposedin the related art (for example, see JP2005-92967A).

SUMMARY OF THE INVENTION

However, data recorded on various recording media such as a magneticrecording medium is called hot data, warm data, and cold data dependingon access frequencies (reproducing frequencies). The access frequenciesdecrease in the order of hot data, warm data, and cold data, and it isgeneral that the cold data is stored as being recorded on a recordingmedium for a long period of time which is longer than 10 years (forexample, several tens of years). The recording and storing of the colddata as described above is referred to as “archive”. The data amount ofthe cold data recorded and stored on a magnetic recording mediumincreases in accordance with a dramatic increase in information contentsand digitization of various information in recent years, andaccordingly, a magnetic recording and reproducing system suitable forthe archive is gaining attention.

In such a circumstance, a green tape test (GTT) is performed as a testfor a magnetic recording and reproducing device (generally referred toas a “drive”). In the GTT, a particular use aspect for archive, in whichcold data having a low access frequency is reproducing, is assumed, anda plurality of (for example, several hundreds of) new (unused) magneticrecording media are slid with respect to one head while changing themagnetic recording media. Meanwhile, in a head durability test in therelated art, a use aspect with a high access frequency compared to thearchive purpose has been assumed, and accordingly, one magneticrecording medium is normally repeatedly slid on the same magnetic head,without changing the magnetic recording medium to a new product. In sucha durability test in the related art, a surface of a magnetic layer isworn while repeating the sliding, and thus, the head element chippinggradually becomes to hardly occur. With respect to this, in the GTT, thesame head is repeatedly slid on a plurality of new magnetic recordingmedia by changing the magnetic recording medium slid on the head to anew product, and thus, the head is in a severe condition where thechipping significantly easily occurs, compared to the durability test inthe related art. In order to prevent such head element chipping in theGTT, the countermeasure on the head side and the countermeasure on themagnetic recording medium side have been considered. For example, as thecountermeasure on the head side, an increase in thickness of aprotective layer of the head has been considered, but an increase inthickness of the protective layer of the head causes an increase indistance between the surface of the magnetic layer and the reproducingelement of the head, and this may cause spacing loss. With respect tothis, in a case where the countermeasure on the magnetic recordingmedium side for preventing the head element chipping in the GTT can befound, a magnetic recording medium with such a countermeasure may be amagnetic recording medium suitable for a recording medium for archive,in which head element chipping hardly occurs in a use aspect forarchive.

Therefore, an aspect of the invention provides for a magnetic recordingmedium suitable for a recording medium for archive capable of exhibitingexcellent electromagnetic conversion characteristics, specifically, amagnetic recording medium capable of exhibiting excellentelectromagnetic conversion characteristics and preventing occurrence ofhead element chipping in a green tape test (GTT).

According to an aspect of the invention, there is provided a magneticrecording medium comprising: a non-magnetic support; and a magneticlayer including a ferromagnetic powder and a binding agent, in which theferromagnetic powder is a ferromagnetic hexagonal ferrite powder, themagnetic layer includes an oxide abrasive, an intensity ratio(Int(110)/Int(114); hereinafter, also referred to as “X-ray diffraction(XRD) intensity ratio) of a peak intensity Int(110) of a diffractionpeak of a (110) plane with respect to a peak intensity Int(114) of adiffraction peak of a (114) plane of a hexagonal ferrite crystalstructure obtained by an X-ray diffraction analysis of the magneticlayer by using an In-Plane method is 0.5 to 4.0, a vertical squarenessratio of the magnetic recording medium is 0.65 to 1.00, a coefficient offriction measured regarding a base portion of a surface of the magneticlayer is equal to or smaller than 0.30, and an average particle diameterof the oxide abrasive obtained from a secondary ion image obtained byirradiating the surface of the magnetic layer with a focused ion beam(FIB) (hereinafter, also referred to as a “FIB abrasive diameter”) is0.04 μm to 0.08 μm.

In one aspect, the vertical squareness ratio may be 0.65 to 0.90.

In one aspect, the coefficient of friction measured regarding the baseportion of the surface of the magnetic layer is 0.15 to 0.30.

In one aspect, the oxide abrasive may be an alumina powder.

In one aspect, the magnetic recording medium may further comprise anon-magnetic layer including a non-magnetic powder and a binding agentbetween the non-magnetic support and the magnetic layer.

In one aspect, the magnetic recording medium may further comprise a backcoating layer including a non-magnetic powder and a binding agent on asurface of the non-magnetic support opposite to a surface provided withthe magnetic layer.

In one aspect, the magnetic recording medium may be a magnetic tape.

According to another aspect of the invention, there is provided amagnetic recording and reproducing device comprising: the magneticrecording medium; and a magnetic head.

In one aspect, the magnetic head may be a magnetic head includingmagnetoresistive (MR) element.

According to one aspect of the invention, it is possible to provide amagnetic recording medium suitable for archive use, which is capable ofexhibiting excellent electromagnetic conversion characteristics andpreventing occurrence of head element chipping in a green tape test(GTT), and a magnetic recording and reproducing device including thismagnetic recording medium.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Magnetic Recording Medium

One aspect of the invention relates to a magnetic recording mediumincluding: a non-magnetic support; and a magnetic layer including aferromagnetic powder and a binding agent, in which the ferromagneticpowder is a ferromagnetic hexagonal ferrite powder, the magnetic layerincludes an oxide abrasive, an intensity ratio (Int(110)/Int(114)) of apeak intensity Int(110) of a diffraction peak of a (110) plane withrespect to a peak intensity Int(114) of a diffraction peak of a (114)plane of a hexagonal ferrite crystal structure obtained by an X-raydiffraction analysis of the magnetic layer by using an In-Plane methodis 0.5 to 4.0, a vertical squareness ratio of the magnetic recordingmedium is 0.65 to 1.00, a coefficient of friction measured regarding abase portion of a surface of the magnetic layer is equal to or smallerthan 0.30, and an average particle diameter of the oxide abrasiveobtained from a secondary ion image obtained by irradiating the surfaceof the magnetic layer with a focused ion beam (FIB abrasive diameter) is0.04 μm to 0.08 μm.

In the invention and the specification, the “surface of the magneticlayer” is identical to the surface of the magnetic recording medium onthe magnetic layer side. In the invention and the specification, the“ferromagnetic hexagonal ferrite powder” means an aggregate of aplurality of ferromagnetic hexagonal ferrite particles. Theferromagnetic hexagonal ferrite particles are ferromagnetic particleshaving a hexagonal ferrite crystal structure. Hereinafter, particles(ferromagnetic hexagonal ferrite particles) configuring theferromagnetic hexagonal ferrite powder are also referred to as“hexagonal ferrite particles” or simply “particles”. The “aggregate” notonly includes an aspect in which particles configuring the aggregate aredirectly in contact with each other, but also includes an aspect inwhich a binding agent or an additive is interposed between theparticles. The points described above are also applied to variouspowders such as non-magnetic powder of the invention and thespecification, in the same manner.

In the invention and the specification, the “oxide abrasive” means anon-magnetic oxide powder having Mohs hardness exceeding 8.

In the invention and the specification, the description regardingdirections and angles (for example, vertical, orthogonal, parallel, andthe like) includes a range of errors allowed in the technical field ofthe invention, unless otherwise noted. For example, the range of errorsmeans a range of less than ±10° from an exact angle, and is preferablywithin ±5° and more preferably within ±3° from an exact angle.

A surmise of the inventors regarding the magnetic recording medium is asfollows.

The inventors have thought that the vertical squareness ratio of themagnetic recording medium and the XRD intensity ratio set to be in theranges described above mainly contribute to the magnetic recordingmedium to exhibit excellent electromagnetic conversion characteristics,specifically to reproduce information recorded on the magnetic recordingmedium at a high signal-to-noise-ratio (SNR). This point will be furtherdescribed hereinafter.

The magnetic recording medium includes the ferromagnetic hexagonalferrite powder in the magnetic layer. The inventors have surmised thatthe ferromagnetic hexagonal ferrite powder included in the magneticlayer includes particles which affect magnetic properties of theferromagnetic hexagonal ferrite powder (aggregate of particles)(hereinafter, also referred to as “former particles”) and particleswhich are considered not to affect or slightly affects the magneticproperties thereof (hereinafter, also referred to as “latterparticles”). It is considered that the latter particles are, forexample, fine particles generated due to partial chipping of particlesdue to a dispersion process performed at the time of preparing amagnetic layer forming composition.

The inventors have thought that, in the particles included in theferromagnetic hexagonal ferrite powder included in the magnetic layer,the former particles are particles causing the diffraction peak in theX-ray diffraction analysis using the In-Plane method, and since thelatter particles are fine, the latter particles do not cause thediffraction peak or hardly affect the diffraction peak. Accordingly, itis surmised that it is possible to control a presence state of theparticles affecting the magnetic properties of the ferromagnetichexagonal ferrite powder present in the magnetic layer, based on theintensity of the diffraction peak caused by the X-ray diffractionanalysis of the magnetic layer using the In-Plane method. The inventorshave surmised that the XRD intensity ratio which will be described laterin detail is an index regarding this point.

Meanwhile, the vertical squareness ratio is a ratio of residualmagnetization with respect to saturation magnetization measured in adirection vertical to the surface of the magnetic layer and this valuedecreases, as a value of the residual magnetization decreases. It issurmised that, since the latter particles are fine and hardly holdmagnetization, as a large amount of the latter particles is included inthe magnetic layer, the vertical squareness ratio tends to decrease.Accordingly, the inventors have thought that the vertical squarenessratio may be an index for the amount of the latter particles (fineparticles) present in the magnetic layer. In addition, the inventorshave thought that, as the amount of such fine particles present in themagnetic layer is small, the magnetic properties of the ferromagnetichexagonal ferrite powder are improved.

In addition, the inventors have surmised that it is possible to improveelectromagnetic conversion characteristics, by setting the verticalsquareness ratio of the magnetic recording medium and the XRD intensityratio to be in the ranges described above, by decreasing the amount oflatter particles (fine particles) present in the magnetic layer andcontrolling the state of the former particles present in the magneticlayer.

Further, the inventors have thought that, the coefficient of frictionmeasured regarding the base portion of the surface of the magnetic layerand the FIB abrasive diameter in the magnetic recording medium set to bein the respective ranges described above mainly contribute to preventionof occurrence of the head element chipping in the GTT. This point willbe further described hereinafter.

The “base portion” of the invention and the specification is a portionof the surface of the magnetic layer of the magnetic recording mediumspecified by the following method.

A surface on which volume of a protruded component and volume of arecess component in a visual field measured by an atomic forcemicroscope (AFM) are identical to each other is determined as areference surface. A projection having a height equal to or greater than15 nm from the reference surface is defined as a projection. A portionin which the number of such projections is zero, that is, a portion ofthe surface of the magnetic layer of the magnetic recording medium inwhich a projection having a height equal to or greater than 15 nm fromthe reference surface is not detected is specified as the base portion.

A coefficient of friction measured regarding the base portion is a valuemeasured by the following method.

In the base portion (measured part: length of a magnetic tape in alongitudinal direction of 10 μm or length of a magnetic disk in a radiusdirection of 10 μm), a diamond spherical indenter having a radius of 1μm is allowed to reciprocate once with a load of 100 μN and a speed of 1μm/sec to measure a frictional force (horizontal force) and a normalforce. The frictional force and the normal force measured here are anarithmetical mean of respective values obtained by continuouslymeasuring frictional forces and normal forces during the onereciprocating operation. The measurement described above can beperformed with TI-950 type TRIBOINDENTER manufactured by Hysitron, Inc.A value of a coefficient of friction μ is calculated from anarithmetical mean of the frictional forces and an arithmetical mean ofthe normal forces measured as described above. The coefficient offriction is a value measured by an equation of F=μN, from the frictionalforce (horizontal force) F (unit: newton (N)) and the normal force N(unit: newton (N)). The measurement and the calculation of the value ofthe coefficient of friction μ are performed at three portions of thebase portion randomly selected from the surface of the magnetic layer ofthe magnetic recording medium, and an arithmetical mean of the threemeasured values obtained is set as a coefficient of friction measuredregarding the base portion. Hereinafter, the coefficient of frictionmeasured regarding the base portion is also referred to as a “basefriction”.

In recent years, a technology of including a non-magnetic powder such asan oxide abrasive in the magnetic layer of the magnetic recording mediumis widely performed. Such a non-magnetic powder is generally protrudedfrom the surface of the magnetic layer and to form projections, andthereby exhibiting various functions. In general, the coefficient offriction measured regarding the magnetic recording medium is acoefficient of friction measured in a region including such projections.With respect to this, the base friction is measured in a portion of thesurface of the magnetic layer of the magnetic recording medium in whicha projection having a height equal to or greater than 15 nm from thereference surface is not detected, that is, the base portion, asdescribed above. It is considered that this base portion has a lowfrequency of a contact with the head in a case where the surface of themagnetic layer and the head slide on each other. However, it is thoughtthat, a high coefficient of friction of the base portion which is incontact with the head, even in a case of a low frequency, disturbssmooth sliding between the base portion and the head. It is thought thatunsmooth sliding between the base portion and head may cause chipping ofthe head element due to the sliding with the surface of the magneticlayer in GTT. With respect to this, it is surmised that, the basefriction in the magnetic recording medium equal to or smaller than 0.30contributes to the smooth sliding between the base portion and the head,thereby contributing to prevention of the occurrence of the head elementchipping in GTT.

In the invention and the specification, the FIB abrasive diameter is avalue obtained by the following method.

(1) Obtaining Secondary Ion Image

A secondary ion image of a region, having a size of 25 μm×25 μm, of thesurface of the magnetic layer of the magnetic recording medium which isa target for obtaining the FIB abrasive diameter is obtained by afocused ion beam device. As the focused ion beam device, MI4050manufactured by Hitachi High-Technologies Corporation can be used.

Beam irradiation conditions of the focused ion beam device in a case ofobtaining the secondary ion image are set so that an accelerationvoltage is 30 kV, a current value is 133 pA (picoampere), a beam size is30 nm, and a brightness is 50%. A coating process with respect to asurface of a magnetic layer before the imaging is not performed. Asecondary ion (SI) signal is detected by a secondary ion detector and asecondary ion image is captured. Conditions for capturing a secondaryion image are determined by the following method. ACB (auto contrastbrightness) is carried out at three spots on a non-imaged region of thesurface of the magnetic layer (i.e., ACB is carried out three times) tostabilize a color of the image. Then, the contrast reference value andthe brightness reference value are determined. The brightness referencevalue as determined in the above ACB and the contrast value which islowered by 1% from the contrast reference value as determined in theabove ACB are determined as the conditions for capturing a secondary ionimage. A non-imaged region of the surface of the magnetic layer isselected, and a secondary ion image is captured under the conditions forcapturing as determined above. A portion for displaying a size and thelike (micron bar, cross mark, and the like) is removed from the capturedimage, and a secondary ion image having the pixel number of 2,000pixel×2,000 pixel is obtained. For specific examples of the imagingconditions, examples which will be described later can be referred to.

(2) Calculation of FIB Abrasive Diameter

The secondary ion image obtained in (1) is put into image processingsoftware, and a binarization process is performed by the followingprocedure. As the image analysis software, ImageJ which is free softwarecan be used, for example.

A tone of the secondary ion image obtained in (1) is changed to 8 bit.Regarding threshold values for the binarization process, a lower limitvalue is set as 250 gradations, an upper limit value is set as 255gradations, and the binarization process is executed by these twothreshold values. After the binarization process, a noise componentremoval process is performed by the image analysis software. The noisecomponent removal process can be carried out, for example, by thefollowing method. On the image analysis software, ImageJ, a noise cutprocess Despeckle is selected, and Size 4.0-Infinity is set onAnalyzeParticle to remove noise components.

Each white-shining portion in the binarization process image obtained asdescribed above is determined as an oxide abrasive, and the number ofwhite-shining portions is obtained by the image analysis software, andthe area of the white-shining portion is obtained. An equivalent circlediameter of each portion is obtained from the area of the white-shiningportion obtained here. Specifically, an equivalent circle diameter L iscalculated from the obtained area A by (A/π){circumflex over( )}(½)×2=L.

The above step is performed four times at different portions (25 μm×25μm) of the surface of the magnetic layer of the magnetic recordingmedium which is a target for obtaining the FIB abrasive diameter, andthe FIB abrasive diameter is calculated from the obtained results by anexpression; FIB abrasive diameter=Σ(Li)/Σi. Σi is a total number of thewhite-shining portions observed in the binarization process imageobtained by performing the above step four times. Σ(Li) is a total ofthe equivalent circle diameters L obtained regarding the white-shiningportions observed in the binarization process image obtained byperforming the above step four times. Regarding the white-shiningportion, only a part of the portion may be included in the binarizationprocess image. In such a case, Σi and Σ(Li) are obtained withoutincluding the part.

The FIB abrasive diameter is a value which can be an index of a presencestate of an oxide abrasive in the magnetic layer, and is obtained fromthe secondary ion image obtained by irradiating the surface of themagnetic layer with a focused ion beam (FIB). This secondary ion imageis generated by capturing secondary ion generated from the surface ofthe magnetic layer irradiated with the FIB. Meanwhile, as an observationmethod of the presence state of the abrasive in the magnetic layer, amethod using a scanning electron microscope (SEM) has been proposed inthe related art. By the SEM, the surface of the magnetic layer isirradiated with an electron beam and secondary electrons emitted fromthe surface of the magnetic layer are captured to generate an image (SEMimage). Even in a case where the same magnetic layer is observed, a sizeof the oxide abrasive obtained from the secondary ion image and a sizeof the oxide abrasive obtained from the SEM image are different fromeach other due to a difference of such image generation principle. As aresult of intensive studies of the inventors, a presence state of theoxide abrasive in the magnetic layer is controlled so that the FIBabrasive diameter becomes 0.04 μm to 0.08 μm, by setting the FIBabrasive diameter obtained from the secondary ion image by the methoddescribed above as a new index of the presence state of the oxideabrasive in the magnetic layer. The inventors have thought that thecontrolling of the presence state of the oxide abrasive in the magneticlayer as described above also contributes to prevention of chipping ofthe head element due to the sliding on the surface of the magnetic layerin the GTT.

The inventors have surmised that, as described above, excellentelectromagnetic conversion characteristics exhibited by the magneticrecording medium mainly contributes to the setting of the XRD intensityratio and the vertical squareness ratio to be in the ranges describedabove, and the prevention of the occurrence of the head element chippingin the GTT mainly contributes to the setting of the base friction andthe FIB abrasive diameter to be in the ranges described above. However,the invention is not limited to the surmise described above.

Hereinafter, the magnetic recording medium will be further described indetail.

XRD Intensity Ratio

The magnetic recording medium includes ferromagnetic hexagonal ferritepowder in the magnetic layer. The XRD intensity ratio is obtained by theX-ray diffraction analysis of the magnetic layer including theferromagnetic hexagonal ferrite powder by using the In-Plane method.Hereinafter, the X-ray diffraction analysis performed by using theIn-Plane method is also referred to as “In-Plane XRD”. The In-Plane XRDis performed by irradiating the surface of the magnetic layer with theX-ray by using a thin film X-ray diffraction device under the followingconditions. The magnetic recording medium is widely divided into atape-shaped magnetic recording medium (magnetic tape) and a disk-shapedmagnetic recording medium (magnetic disk). A measurement direction is alongitudinal direction of the magnetic tape and a radius direction ofthe magnetic disk.

Cu ray source used (output of 45 kV, 200 mA)

Scan conditions: 0.05 degree/step, 0.1 degree/min in a range of 20 to 40degrees

Optical system used: parallel optical system

Measurement method: 2θ_(χ) scan (X-ray incidence angle of 0.25°)

The values of the conditions are set values of the thin film X-raydiffraction device. As the thin film X-ray diffraction device, awell-known device can be used. As an example of the thin film X-raydiffraction device, Smart Lab manufactured by Rigaku Corporation. Asample to be subjected to the In-Plane XRD analysis is a medium samplecut out from the magnetic recording medium which is a measurementtarget, and the size and the shape thereof are not limited, as long asthe diffraction peak which will be described later can be confirmed.

As a method of the X-ray diffraction analysis, thin film X-raydiffraction and powder X-ray diffraction are used. In the powder X-raydiffraction, the X-ray diffraction of the powder sample is measured,whereas, according to the thin film X-ray diffraction, the X-raydiffraction of a layer or the like formed on a substrate can bemeasured. The thin film X-ray diffraction is classified into theIn-Plane method and an Out-Of-Plane method. The X-ray incidence angle atthe time of the measurement is 5.00° to 90.00° in a case of theOut-Of-Plane method, and is generally 0.20° to 0.50°, in a case of theIn-Plane method. In the In-Plane XRD of the invention and thespecification, the X-ray incidence angle is 0.25° as described above. Inthe In-Plane method, the X-ray incidence angle is smaller than that inthe Out-Of-Plane method, and thus, a depth of penetration of the X-rayis shallow. Accordingly, according to the X-ray diffraction analysis byusing the In-Plane method (In-Plane XRD), it is possible to perform theX-ray diffraction analysis of a surface portion of a measurement targetsample. Regarding the magnetic recording medium sample, according to theIn-Plane XRD, it is possible to perform the X-ray diffraction analysisof the magnetic layer. The XRD intensity ratio is an intensity ratio(Int(110)/Int(114)) of a peak intensity Int(110) of a diffraction peakof a (110) plane with respect to a peak intensity Int(114) of adiffraction peak of a (114) plane of a hexagonal ferrite crystalstructure, in X-ray diffraction spectra obtained by the In-Plane XRD.The term Int is used as abbreviation of intensity. In the X-raydiffraction spectra obtained by In-Plane XRD (vertical axis: intensity,horizontal axis: diffraction angle 2θ_(χ) (degree)), the diffractionpeak of the (114) plane is a peak at which the 2θ_(χ) is detected at 33to 36 degrees, and the diffraction peak of the (110) plane is a peak atwhich the 2θ_(χ) is detected at 29 to 32 degrees.

Among the diffraction plane, the (114) plane having a hexagonal ferritecrystal structure is positioned close to particles of the ferromagnetichexagonal ferrite powder (hexagonal ferrite particles) in aneasy-magnetization axial direction (c axis direction). In addition the(110) plane having a hexagonal ferrite crystal structure is positionedin a direction orthogonal to the easy-magnetization axial direction.

The inventors have surmised that, in the X-ray diffraction spectraobtained by the In-Plane XRD, as the intensity ratio (Int(110)/Int(114);XRD intensity ratio) of the peak intensity Int(110) of the diffractionpeak of a (110) plane with respect to the peak intensity Int(114) of thediffraction peak of the (114) plane of a hexagonal ferrite crystalstructure increases, a large number of the former particles present in astate where a direction orthogonal to the easy-magnetization axialdirection is closer to a parallel state with respect to the surface ofthe magnetic layer is present in the magnetic layer, and as the XRDintensity ratio decreases, a small amount of the former particlespresent in such a state is present in the magnetic layer. It is thoughtthat a state where the XRD intensity ratio is 0.5 to 4.0 means a statewhere the former particles are suitably aligned in the magnetic layer.The inventors have surmised that this contributes to the improvement ofelectromagnetic conversion characteristics.

The XRD intensity ratio is preferably equal to or smaller than 3.5 andmore preferably equal to or smaller than 3.0, from a viewpoint offurther improving electromagnetic conversion characteristics. From thesame viewpoint, the XRD intensity ratio is preferably equal to orgreater than 0.7 and more preferably equal to or greater than 1.0. TheXRD intensity ratio can be, for example, controlled in accordance withprocess conditions of an alignment process performed in a manufacturingstep of the magnetic recording medium. As the alignment process, thehomeotropic alignment process is preferably performed. The homeotropicalignment process can be preferably performed by applying a magneticfield vertically to the surface of a coating layer of a magnetic layerforming composition in a wet state (undried state). As the alignmentconditions are reinforced, the value of the XRD intensity ratio tends toincrease. As the process conditions of the alignment process, magneticfield strength of the alignment process is used. The process conditionsof the alignment process are not particularly limited. The processconditions of the alignment process may be set so as that the XRDintensity ratio of 0.5 to 4.0 can be realized. As an example, themagnetic field strength of the homeotropic alignment process can be 0.10to 0.80 T or 0.10 to 0.60 T. As dispersibility of the ferromagnetichexagonal ferrite powder in the magnetic layer forming compositionincreases, the value of the XRD intensity ratio tends to increase by thehomeotropic alignment process.

Vertical Squareness Ratio

The vertical squareness ratio is a squareness ratio measured regarding amagnetic recording medium in a vertical direction. The “verticaldirection” described regarding the squareness ratio is a directionorthogonal to the surface of the magnetic layer. For example, in a casewhere the magnetic recording medium is a tape-shaped magnetic recordingmedium, that is, a magnetic tape, the vertical direction is a directionorthogonal to a longitudinal direction of the magnetic tape. Thevertical squareness ratio is measured by using a vibrating samplemagnetometer. Specifically, the vertical squareness ratio of theinvention and the specification is a value obtained by sweeping anexternal magnetic field in the magnetic recording medium at ameasurement temperature of 23° C.±1° C. in the vibrating samplemagnetometer, under conditions of a maximum external magnetic field of1194 kA/m (15 kOe) and a scan speed of 4.8 kA/m/sec (60 Oe/sec), and isa value after diamagnetic field correction. The measurement value isobtained as a value obtained by subtracting magnetization of a sampleprobe of the vibrating sample magnetometer as background noise.

The vertical squareness ratio of the magnetic recording medium is equalto or greater than 0.65. The inventors have surmised that the verticalsquareness ratio of the magnetic recording medium is an index for thepresence amount of the latter particles (fine particles) describedabove. It is thought that, in the magnetic layer in which the verticalsquareness ratio of the magnetic recording medium is equal to or greaterthan 0.65, the presence amount of such fine particles is small. Theinventors have surmised that this contributes to the improvement ofelectromagnetic conversion characteristics. From a viewpoint of furtherimproving electromagnetic conversion characteristics, the verticalsquareness ratio is preferably equal to or greater than 0.70, morepreferably equal to or greater than 0.73, and even more preferably equalto or greater than 0.75. In addition, in principle, a maximum value ofthe squareness ratio is 1.00. Accordingly, the vertical squareness ratioof the magnetic tape is equal to or smaller than 1.00. The verticalsquareness ratio may be, for example, equal to or smaller than 0.95,equal to or smaller than 0.90, equal to or smaller than 0.87, or equalto or smaller than 0.85. However, it is thought that, a great value ofthe vertical squareness ratio is preferable, from a viewpoint ofdecreasing the amount of the fine latter particles in the magnetic layerand improving electromagnetic conversion characteristics. Therefore, thevertical squareness ratio may be greater than the value exemplifiedabove.

The inventors have considered that, in order to set the verticalsquareness ratio to be equal to or greater than 0.65, it is preferableto prevent occurrence of fine particles due to partial chipping of theparticles in a preparation step of the magnetic layer formingcomposition. Specific means for preventing the occurrence of chippingwill be described later.

Base Friction

The coefficient of friction (base friction) measured regarding the baseportion of the surface of the magnetic layer of the magnetic recordingmedium is equal to or smaller than 0.30, is preferably equal to orsmaller than 0.28 and more preferably equal to or smaller than 0.26,from a viewpoint of further preventing the occurrence of the headelement chipping in GTT. The base friction can be, for example, equal toor greater than 0.10, equal to or greater than 0.15, or equal to orgreater than 0.20. However, from a viewpoint of preventing theoccurrence of the head element chipping in GTT, a low base friction ispreferable, and thus, the base friction may be smaller than the valuesdescribed above.

In the measurement method of the base friction described above, thereason why a projection having a height equal to or greater than 15 nmfrom the reference surface is defined as a projection is because,normally, a projection recognized as a projection present on the surfaceof the magnetic layer is mainly a projection having a height equal to orgreater than 15 nm from the reference surface. Such a projection is, forexample, formed of non-magnetic powder such as an oxide abrasive on thesurface of the magnetic layer. With respect to this, it is consideredthat more microscopic ruggedness than ruggedness formed by such aprojection is present on the surface of the magnetic layer. It issurmised that it is possible to adjust the base friction by controllinga shape of the microscopic ruggedness. As a method for realizing theadjustment described above, a method of using two or more kinds offerromagnetic hexagonal ferrite powders having different averageparticle sizes as the ferromagnetic hexagonal ferrite powder is used.More specifically, it is thought that, the microscopic ruggedness can beformed on the base portion, in a case where the ferromagnetic hexagonalferrite powder having a greater average particle size becomes aprotruded component, and it is possible to increase a percentage of theprotruded component present on the base portion by increasing a mixingpercentage of the ferromagnetic hexagonal ferrite powder having agreater average particle size (or, conversely, to decrease a percentageof protruded component present on the base portion by decreasing themixing percentage). Such a method will be described later morespecifically.

As another method, a method of forming a magnetic layer with othernon-magnetic powder having a greater average particle size than that offerromagnetic hexagonal ferrite powder, in addition to non-magneticpowder such as an oxide abrasive which can form a projection having aheight equal to or greater than 15 nm from the reference surface on thesurface of the magnetic layer. More specifically, it is thought that,the microscopic ruggedness can be formed on the base portion, in a casewhere the other non-magnetic powder becomes a protruded component, andit is possible to increase a percentage of the protruded componentpresent on the base portion by increasing a mixing percentage of thenon-magnetic powder (or, conversely, to decrease a percentage ofprotruded component present on the base portion by decreasing the mixingpercentage). Such a method will be described later more specifically.

In addition, it is also possible to adjust the base friction bycombining the two kinds of methods.

However, the adjustment methods are merely examples, and it is possibleto realize a base friction equal to or smaller than 0.30 by any methodcapable of adjusting the base friction, and such an aspect is alsoincluded in the invention.

FIB Abrasive Diameter

The FIB abrasive diameter obtained from the secondary ion image obtainedby irradiating the surface of the magnetic layer of the magneticrecording medium with the FIB is 0.04 μm to 0.08 μm. It is thought thatthe FIB abrasive diameter set to be equal to or smaller than 0.08contributes to the prevention of the chipping of the head element due tothe oxide abrasive in the GTT. In addition, it is surmised that the FIBabrasive diameter set to be equal to or greater than 0.04 μm contributesto the removal of a component derived from the magnetic layer attachedto the head due to the sliding with the surface of the magnetic layer inthe GTT. It is thought that this contributes to prevention of thechipping of the element of the head due to the sliding between thesurface of the magnetic layer and the head, in a state where thecomponent derived from the magnetic layer is attached to the head in theGTT. From a viewpoint of further preventing the occurrence of the headelement chipping in the GTT, the FIB abrasive diameter is preferablyequal to or greater than 0.05 μm and more preferably equal to or greaterthan 0.06 μm. In addition, from the same viewpoint, the FIB abrasivediameter is preferably equal to or smaller than 0.07 μm. A specificaspect of means for adjusting the FIB abrasive diameter will bedescribed later.

Hereinafter, the magnetic recording medium will be described morespecifically.

Magnetic Layer

Ferromagnetic Hexagonal Ferrite Powder

The magnetic layer of the magnetic recording medium includesferromagnetic hexagonal ferrite powder as ferromagnetic powder.Regarding the ferromagnetic hexagonal ferrite powder, a magnetoplumbitetype (also referred to as an “M type”), a W type, a Y type, and a Z typeare known as the crystal structure of the hexagonal ferrite. Theferromagnetic hexagonal ferrite powder included in the magnetic layermay have any crystal structure. In addition, an iron atom and a divalentmetal atom are included in the crystal structure of the hexagonalferrite, as constituent atoms. The divalent metal atom is a metal atomwhich may become divalent cations as ions, and examples thereof includea barium atom, a strontium atom, an alkali earth metal atom such ascalcium atom, and a lead atom. For example, the hexagonal ferriteincluding a barium atom as the divalent metal atom is a barium ferrite,and the hexagonal ferrite including a strontium atom is a strontiumferrite. In addition, the hexagonal ferrite may be a mixed crystal oftwo or more hexagonal ferrites. As an example of the mixed crystal, amixed crystal of the barium ferrite and the strontium ferrite can beused.

As described above, as a method of adjusting the base friction, a methodof forming a magnetic layer with two or more kinds of ferromagnetichexagonal ferrite powders having different average particle sizes asferromagnetic hexagonal ferrite powder is used. In this case, it ispreferable that the ferromagnetic powder having a smaller averageparticle size is used as ferromagnetic hexagonal ferrite powder usedwith the largest proportion, among the two or more kinds offerromagnetic hexagonal ferrite powder, from a viewpoint of improvingrecording density of the magnetic recording medium. From this viewpoint,in a case where two or more kinds of ferromagnetic hexagonal ferritepowders having different average particle sizes are included in themagnetic layer, it is preferable that the ferromagnetic hexagonalferrite powder having an average particle size equal to or smaller than50 nm is preferably used as the ferromagnetic hexagonal ferrite powderwith the largest proportion based on mass. On the other hand, it ispreferable that an average particle size of the ferromagnetic hexagonalferrite powder with the largest proportion is equal to or greater than10 nm, from a viewpoint of stability of magnetization. In a case ofusing one kind of ferromagnetic hexagonal ferrite powder without usingtwo or more kinds of ferromagnetic hexagonal ferrite powders havingdifferent average particle sizes, the average particle size of theferromagnetic hexagonal ferrite powder is preferably 10 nm to 50 nm, dueto the reasons described above.

With respect to this, it is preferable that the ferromagnetic hexagonalferrite powder used with the ferromagnetic hexagonal ferrite powder withthe largest proportion preferably has a greater average particle sizethan that of the ferromagnetic hexagonal ferrite powder with the largestproportion. This may be because the base friction can be decreased dueto the protruded component formed of the ferromagnetic hexagonal ferritepowder having a great average particle size on the base portion. Fromthis viewpoint, a difference between an average particle size of theferromagnetic hexagonal ferrite powder with the largest proportion andan average particle size of the ferromagnetic hexagonal ferrite powderused therewith, acquired as “(latter average particle size)−(formeraverage particle size)” is preferably 10 to 80 nm, more preferably 10 to50 nm, and even more preferably 10 to 40 nm. As the ferromagnetichexagonal ferrite powder used with the ferromagnetic hexagonal ferritepowder with the largest proportion, it is also possible to use two ormore kinds of ferromagnetic hexagonal ferrite powders having differentaverage particle sizes. In this case, it is preferable that an averageparticle size of at least one of ferromagnetic hexagonal ferrite powderof the two or more kinds of ferromagnetic hexagonal ferrite powderssatisfies the difference described above, it is more preferable thataverage particle sizes of more kinds of ferromagnetic hexagonal ferritepowders satisfy the difference described above, and it is even morepreferable that average particle sizes of all of the ferromagnetichexagonal ferrite powders satisfy the difference described above, withrespect to the average particle size of the ferromagnetic hexagonalferrite powder used with the largest proportion.

Regarding two or more kinds of ferromagnetic hexagonal ferrite powdershaving different average particle sizes, from a viewpoint of controllingbase friction, a mixing ratio of the ferromagnetic hexagonal ferritepowder with the largest proportion to the other ferromagnetic powder (ina case of using two or more kinds of ferromagnetic hexagonal ferritepowders having different average particle sizes as other ferromagnetichexagonal ferrite powder, the total thereof), is preferably 90.0:10.0(former:latter) to 99.9:0.1 and more preferably 95.0:5.0 to 99.5:0.5based on mass.

Here, the ferromagnetic hexagonal ferrite powders having differentaverage particle sizes indicate the total or a part of a batch of theferromagnetic hexagonal ferrite powders having different averageparticle sizes. In a case where particle size distribution based on thenumber or volume of the ferromagnetic hexagonal ferrite powder includedin the magnetic layer of the magnetic recording medium formed with theferromagnetic hexagonal ferrite powders having different averageparticle sizes as described above is measured by a well-knownmeasurement method such as a dynamic light scattering method or a laserdiffraction method, an average particle size or a maximum peak in thevicinity thereof of the ferromagnetic hexagonal ferrite powder used withthe largest proportion can be normally confirmed in a particle sizedistribution curve obtained by the measurement. In addition, an averageparticle size or a peak in the vicinity thereof of each ferromagnetichexagonal ferrite powder may be confirmed. Accordingly, in a case wherethe particle size distribution of the ferromagnetic hexagonal ferritepowder included in the magnetic layer of the magnetic recording mediumformed by using ferromagnetic hexagonal ferrite powder having an averageparticle size of 10 to 50 nm with the largest proportion, for example,is measured, the maximum peak can be generally confirmed in a range ofthe particle size of 10 to 50 nm in the particle size distributioncurve.

A part of the other ferromagnetic hexagonal ferrite powders describedabove may be substituted with other non-magnetic powder which will bedescribed later.

In the invention and the specification, average particle sizes ofvarious powder such as the ferromagnetic hexagonal ferrite powder andthe like are values measured by the following method with a transmissionelectron microscope, unless otherwise noted.

The powder is imaged at a magnification ratio of 100,000 with atransmission electron microscope, the image is printed on printing paperso that the total magnification of 500,000 to obtain an image ofparticles configuring the powder. A target particle is selected from theobtained image of particles, an outline of the particle is traced with adigitizer, and a size of the particle (primary particle) is measured.The primary particle is an independent particle which is not aggregated.

The measurement described above is performed regarding 500 particlesrandomly extracted. An arithmetical mean of the particle size of 500particles obtained as described above is an average particle size of thepowder. As the transmission electron microscope, a transmission electronmicroscope H-9000 manufactured by Hitachi, Ltd. can be used, forexample. In addition, the measurement of the particle size can beperformed by well-known image analysis software, for example, imageanalysis software KS-400 manufactured by Carl Zeiss.

In the invention and the specification, the average particle size of theferromagnetic hexagonal ferrite powder and other powder is an averageparticle size obtained by the method described above, unless otherwisenoted. The average particle size shown in examples which will bedescribed later is a value measured by using transmission electronmicroscope H-9000 manufactured by Hitachi, Ltd. as the transmissionelectron microscope, and image analysis software KS-400 manufactured byCarl Zeiss as the image analysis software, unless otherwise noted.

As a method of collecting a sample powder from the magnetic tape inorder to measure the particle size, a method disclosed in a paragraph of0015 of JP2011-048878A can be used, for example.

In the invention and the specification, unless otherwise noted,

(1) in a case where the shape of the particle observed in the particleimage described above is a needle shape, a fusiform shape, or a columnarshape (here, a height is greater than a maximum long diameter of abottom surface), the size (article size) of the particles configuringthe powder is shown as a length of a long axis configuring the particle,that is, a long axis length,

(2) in a case where the shape of the particle is a planar shape or acolumnar shape (here, a thickness or a height is smaller than a maximumlong diameter of a plate surface or a bottom surface), the particle sizeis shown as a maximum long diameter of the plate surface or the bottomsurface, and

(3) in a case where the shape of the particle is a sphere shape, apolyhedron shape, or an unspecified shape, and the long axis configuringthe particles cannot be specified from the shape, the particle size isshown as an equivalent circle diameter. The equivalent circle diameteris a value obtained by a circle projection method.

In addition, regarding an average acicular ratio of the powder, a lengthof a short axis, that is, a short axis length of the particles ismeasured in the measurement described above, a value of (long axislength/short axis length) of each particle is obtained, and anarithmetical mean of the values obtained regarding 500 particles iscalculated. Here, unless otherwise noted, in a case of (1), the shortaxis length as the definition of the particle size is a length of ashort axis configuring the particle, in a case of (2), the short axislength is a thickness or a height, and in a case of (3), the long axisand the short axis are not distinguished, thus, the value of (long axislength/short axis length) is assumed as 1, for convenience.

In addition, unless otherwise noted, in a case where the shape of theparticle is specified, for example, in a case of definition of theparticle size (1), the average particle size is an average long axislength, and in a case of the definition (2), the average particle sizeis an average plate diameter. In a case of the definition (3), theaverage particle size is an average diameter (also referred to as anaverage particle diameter).

The shape of the particle configuring the ferromagnetic hexagonalferrite powder is obtained by tracing an outline of the particle(primary particle) with a digitizer in the image of the particleobtained by using the transmission electron microscope as describedabove. Regarding the shape of the particle configuring the ferromagnetichexagonal ferrite powder, a “planar shape” is a shape having two platesurfaces facing each other. Meanwhile, among the shapes of the particlesnot having such a plate surface, a shape having distinguished long axisand short axis is an “elliptical shape”. The long axis is determined asan axis (linear line) having the longest length of the particle. Incontrast, the short axis is determined as an axis having the longestlength of the particle in a linear line orthogonal to the long axis. Ashape not having distinguished long axis and short axis, that is, ashape in which the length of the long axis is the same as the length ofthe short axis is a “sphere shape”. From the shapes, a shape in whichthe long axis and the short axis are hardly specified, is called anundefined shape. The imaging with a transmission electron microscope forspecifying the shapes of the particles is performed without performingthe alignment process with respect to the imaging target powder. Theshape of the raw material powder used for the preparation of themagnetic layer forming composition and the ferromagnetic hexagonalferrite powder included in the magnetic layer may be any one of theplanar shape, the elliptical shape, the sphere shape, and the undefinedshape.

For details of the ferromagnetic hexagonal ferrite powder, descriptionsdisclosed in paragraphs 0134 to 0136 of JP2011-216149A can be referredto, for example.

The content (filling percentage) of the ferromagnetic hexagonal ferritepowder of the magnetic layer is preferably 50% to 90% by mass and morepreferably 60% to 90% by mass. The components other than theferromagnetic hexagonal ferrite powder of the magnetic layer are atleast a binding agent and an oxide abrasive, and one or more kinds ofadditives can be randomly included. A high filling percentage of theferromagnetic hexagonal ferrite powder of the magnetic layer ispreferable, from a viewpoint of improving recording density.

Binding Agent and Curing Agent

The magnetic recording medium includes a binding agent in the magneticlayer. The binding agent is one or more kinds of resin. The resin may bea homopolymer or a copolymer. As the binding agent included in themagnetic layer, a resin selected from a polyurethane resin, a polyesterresin, a polyamide resin, a vinyl chloride resin, an acrylic resinobtained by copolymerizing styrene, acrylonitrile, or methylmethacrylate, a cellulose resin such as nitrocellulose, an epoxy resin,a phenoxy resin, and a polyvinylalkylal resin such as polyvinyl acetalor polyvinyl butyral can be used alone or a plurality of resins can bemixed with each other to be used. Among these, a polyurethane resin, anacrylic resin, a cellulose resin, and a vinyl chloride resin arepreferable. These resins can be used as the binding agent even in thenon-magnetic layer and/or a back coating layer which will be describedlater. For the binding agent described above, description disclosed inparagraphs 0029 to 0031 of JP2010-24113A can be referred to. An averagemolecular weight of the resin used as the binding agent can be, forexample, 10,000 to 200,000 as a weight-average molecular weight. Theweight-average molecular weight in the invention and the specificationis a value obtained by performing polystyrene conversion of a valuemeasured by gel permeation chromatography (GPC), unless otherwise noted.As the measurement conditions, the following conditions can be used. Theweight-average molecular weight shown in examples which will bedescribed later is a value obtained by performing polystyrene conversionof a value measured under the following measurement conditions.

GPC device: HLC-8120 (manufactured by Tosoh Corporation)

Column: TSK gel Multipore HXL-M (manufactured by Tosoh Corporation, 7.8mmID (inner diameter)×30.0 cm)

Eluent: Tetrahydrofuran (THF)

In addition, a curing agent can also be used together with the resinwhich can be used as the binding agent, in a case of forming themagnetic layer. As the curing agent, in one aspect, a thermosettingcompound which is a compound in which a curing reaction (crosslinkingreaction) proceeds due to heating can be used, and in another aspect, aphotocurable compound in which a curing reaction (crosslinking reaction)proceeds due to light irradiation can be used. At least a part of thecuring agent is included in the magnetic layer in a state of beingreacted (crosslinked) with other components such as the binding agent,by proceeding the curing reaction in the manufacturing step of themagnetic recording medium. The preferred curing agent is a thermosettingcompound, polyisocyanate is suitable. For details of the polyisocyanate,descriptions disclosed in paragraphs 0124 and 0125 of JP2011-216149A canbe referred to, for example. The amount of the curing agent added andused can be, for example, 0 to 80.0 parts by mass with respect to 100.0parts by mass of the binding agent in the magnetic layer formingcomposition, and is preferably 50.0 to 80.0 parts by mass, from aviewpoint of improvement of hardness of the magnetic layer.

Oxide Abrasive

The magnetic recording medium includes an oxide abrasive in the magneticlayer. The oxide abrasive is a non-magnetic oxide powder having Mohshardness exceeding 8 and is preferably a non-magnetic oxide powderhaving Mohs hardness equal to or greater than 9. A maximum value of Mohshardness is 10. The oxide abrasive may be an inorganic oxide powder andan organic oxide powder, and is preferably an inorganic oxide powder.Specifically, examples of the abrasive include powders of alumina(Al₂O₃), titanium oxide (TiO₂), cerium oxide (CeO₂), and zirconium oxide(ZrO₂), and alumina powder is preferable among these. Mohs hardness ofalumina is approximately 9. For alumina powder, a description disclosedin a paragraph 0021 of JP2013-229090A can also be referred to. Inaddition, as an index of a particle size of the oxide abrasive, aspecific surface area can be used. It is thought that, as the specificsurface area is great, the particle size of the primary particles of theparticles configuring the oxide abrasive is small. As the oxideabrasive, an oxide abrasive in which a specific surface area measured bya Brunauer-Emmett-Teller (BET) method (hereinafter, referred to as a“BET specific surface area”) is equal to or greater than 14 m²/g, ispreferably used. In addition, from a viewpoint of dispersibility, anoxide abrasive having a BET specific surface area equal to or smallerthan 40 m²/g is preferably used. The content of the oxide abrasive inthe magnetic layer is preferably 1.0 to 20.0 parts by mass and morepreferably 1.0 to 10.0 parts by mass with respect to 100.0 parts by massof the ferromagnetic hexagonal ferrite powder.

Additives

The magnetic layer includes ferromagnetic hexagonal ferrite powder, abinding agent and an oxide abrasive, and may further include one or morekinds of additives, if necessary. As the additives, the curing agentdescribed above is used as an example. In addition, examples of theadditive which can be included in the magnetic layer include anon-magnetic powder other than the oxide abrasive, a lubricant, adispersing agent, a dispersing assistant, an antibacterial agent, anantistatic agent, and an antioxidant. As the additives, a commerciallyavailable product can be suitably selected or manufactured by awell-known method according to the desired properties, and any amountthereof can be used. For example, for the lubricant, a descriptiondisclosed in paragraphs 0030 to 0033, 0035, and 0036 of JP2016-126817Acan be referred to. The lubricant may be included in the non-magneticlayer. For the lubricant which can be included in the non-magneticlayer, a description disclosed in paragraphs 0030, 0031, 0034, 0035, and0036 of JP2016-126817A can be referred to. For the dispersing agent, adescription disclosed in paragraphs 0061 and 0071 of JP2012-133837A canbe referred to. The dispersing agent may be included in the non-magneticlayer. For the dispersing agent which can be included in thenon-magnetic layer, a description disclosed in a paragraph 0061 ofJP2012-133837A can be referred to.

In addition, as the dispersing agent, a dispersing agent for increasingdispersibility of the oxide abrasive can be used. As a compound whichcan function as such a dispersing agent, an aromatic hydrocarboncompound including a phenolic hydroxyl group can be used. The “phenolichydroxyl group” is a hydroxyl group directly bonded to an aromatic ring.The aromatic ring included in the aromatic hydrocarbon compound may be amonocycle, may have a polycyclic structure, or may be a condensed ring.From a viewpoint of improving dispersibility of the abrasive, anaromatic hydrocarbon compound including a benzene ring or a naphthalenering is preferable. In addition, the aromatic hydrocarbon compound mayinclude a substituent other than the phenolic hydroxyl group. Examplesof the substituent other than the phenolic hydroxyl group include ahalogen atom, an alkyl group, an alkoxy group, an amino group, an acylgroup, a nitro group, a nitroso group, and a hydroxyalkyl group, and ahalogen atom, an alkyl group, an alkoxy group, an amino group, and ahydroxyalkyl group are preferable. The number of phenolic hydroxylgroups included in one molecule of the aromatic hydrocarbon compound maybe one, two, three, or greater.

As a preferable aspect of the aromatic hydrocarbon compound includingthe phenolic hydroxyl group, a compound represented by General Formula100 can be used.

[In General Formula 100, two of X¹⁰¹ to X¹⁰⁸ are hydroxyl groups and theother six thereof each independently represent a hydrogen atom or asubstituent.]

In the compound represented by General Formula 100, substituentpositions of the two hydroxyl groups (phenolic hydroxyl groups) are notparticularly limited.

In the compound represented by General Formula 100, two of X¹⁰¹ to X¹⁰⁸are hydroxyl groups (phenolic hydroxyl groups) and the other six thereofeach independently represent a hydrogen atom or a substituent. Inaddition, among X¹⁰¹ to X¹⁰⁸, all of the part other than the twohydroxyl groups may be a hydrogen atom or a part or all thereof may be asubstituent. As the substituent, the substituent described above can beused. As the substituent other than the two hydroxyl groups, one or morephenolic hydroxyl groups may be included. From a viewpoint of improvingdispersibility of the abrasive, it is preferable that the substituentother than the two hydroxyl groups of X¹⁰¹ to X¹⁰⁸ is not a phenolichydroxyl group. That is, the compound represented by General Formula 100is preferably dihydroxynaphthalene or a derivative thereof, and morepreferably 2,3-dihydroxynaphthalene or a derivative thereof. Examples ofthe preferable substituent represented by X¹⁰¹ to X¹⁰⁸ include a halogenatom (for example, a chlorine atom or a bromine atom), an amino group,an alkyl group having 1 to 6 (preferably 1 to 4) carbon atoms, a methoxygroup, an ethoxy group, an acyl group, a nitro group, a nitroso group,and a —CH₂OH group.

In addition, for the dispersing agent for increasing dispersibility ofthe oxide abrasive, a description disclosed in paragraphs 0024 to 0028of JP2014-179149A can be referred to.

The used amount of the dispersing agent for increasing dispersibility ofthe oxide abrasive can be, for example, 0.5 to 20.0 parts by mass and ispreferably 1.0 to 10.0 parts by mass with respect to 100.0 parts by massof the abrasive in a case of preparing a magnetic layer formingcomposition (preferably, in a case of preparing an abrasive solution aswill be described later).

As the dispersing agent, a well-known dispersing agent for increasingdispersibility of ferromagnetic hexagonal ferrite powder such as acarboxyl group-containing compound or a nitrogen-containing compound canalso be used. For example, the nitrogen-containing compound may primaryamine represented by NH₂R, secondary amine represented by NHR₂, ortertiary amine represented by NR₃. As described above, R indicates anystructure configuring the nitrogen-containing compound and a pluralityof R may be the same as each other or different from each other. Thenitrogen-containing compound may be a compound (polymer) having aplurality of repeating structures in a molecule. It is thought that anitrogen-containing portion of the nitrogen-containing compoundfunctioning as an adsorption portion to the surface of the particles ofthe ferromagnetic hexagonal ferrite powder is a reason for thenitrogen-containing compound to function as the dispersing agent. As thecarboxyl group-containing compound, for example, fatty acid of oleicacid can be used. Regarding the carboxyl group-containing compound, itis thought that a carboxyl group functioning as an adsorption portion tothe surface of the particles of the ferromagnetic hexagonal ferritepowder is a reason for the carboxyl group-containing compound tofunction as the dispersing agent. It is also preferable to use thecarboxyl group-containing compound and the nitrogen-containing compoundin combination. The amount of these dispersing agents used can besuitably set.

As the non-magnetic powder other than the oxide abrasive included in themagnetic layer, non-magnetic powder which can contribute to formation ofprojections on the surface of the magnetic layer to control of frictionproperties (hereinafter, also referred to as a “projection formationagent”). As the projection formation agent, various non-magnetic powdersgenerally used as the projection formation agent in the magnetic layercan be used. These may be powder of inorganic substance (inorganicpowder) or powder of organic substance (organic powder). In one aspect,from a viewpoint of homogenization of friction properties, particle sizedistribution of the projection formation agent is not polydispersionhaving a plurality of peaks in the distribution and is preferablymonodisperse showing a single peak. From a viewpoint of availability ofmonodisperse particles, the projection formation agent is preferablyinorganic powder. Examples of the inorganic powder include powder ofmetal oxide, metal carbonate, metal sulfate, metal nitride, metalcarbide, and metal sulfide. The particles configuring the projectionformation agent (non-magnetic powder other than the oxide abrasive) arepreferably colloid particles and more preferably inorganic oxide colloidparticles. In addition, from a viewpoint of availability of monodisperseparticles, the inorganic oxide configuring the inorganic oxide colloidparticles are preferably silicon dioxide (silica). The inorganic oxidecolloid particles are more preferably colloidal silica (silica colloidparticles). In the invention and the specification, the “colloidparticles” are particles which are not precipitated and dispersed togenerate a colloidal dispersion, in a case where 1 g of the particles isadded to 100 mL of at least one organic solvent of methyl ethyl ketone,cyclohexanone, toluene, or ethyl acetate, or a mixed solvent includingtwo or more kinds of the solvent described above at a random mixingratio. In another aspect, the projection formation agent is preferablycarbon black. An average particle size of the projection formation agentcan be, for example, 30 to 300 nm and is preferably 40 to 200 nm. Inaddition, from a viewpoint that the projection formation agent exhibitsthe functions thereof in more excellent manner, the content of theprojection formation agent in the magnetic layer is preferably 1.0 to4.0 parts by mass and more preferably 1.5 to 3.5 parts by mass withrespect to 100.0 parts by mass of the ferromagnetic hexagonal ferritepowder.

As described above, in order to control the base friction to be equal toor smaller than 0.30, other non-magnetic powders can also be used inaddition to the oxide abrasive and the projection formation agentdescribed above. For such non-magnetic powder, various non-magneticpowders normally used in the non-magnetic layer can be used.Specifically, the non-magnetic layer is as described later. As morepreferred non-magnetic powder, red oxide can be used. The Mohs hardnessof red oxide is approximately 6.

It is preferable that the other non-magnetic powder described above hasan average particle size greater than that of the ferromagnetichexagonal ferrite powder, in the same manner as the ferromagnetichexagonal ferrite powder used with the ferromagnetic hexagonal ferritepowder included with the largest proportion described above. This isbecause the base friction may decrease due to the protruded componentformed of the other non-magnetic powder on the base portion. From aviewpoint, difference between an average particle size of theferromagnetic hexagonal ferrite powder and an average particle size ofthe other non-magnetic powder used therewith, acquired as “(latteraverage particle size)−(former average particle size)” is preferably 10to 80 nm and more preferably 10 to 50 nm. In a case of using two or morekinds of ferromagnetic hexagonal ferrite powders having differentaverage particle sizes as the ferromagnetic hexagonal ferrite powder,the ferromagnetic hexagonal ferrite powder used for calculating adifference between the average particle size thereof and the averageparticle size of the other non-magnetic hexagonal ferrite powder is aferromagnetic hexagonal ferrite powder with the largest proportion amongtwo or more kinds of ferromagnetic hexagonal ferrite powders based onmass. As the other non-magnetic powder, it is also possible to use twoor more kinds of non-magnetic powders having different average particlesizes. In this case, it is preferable that an average particle size ofat least one of non-magnetic powder of the two or more of non-magneticpowders satisfies the difference described above, it is more preferablethat average particle sizes of more kinds of non-magnetic powderssatisfy the difference described above, and it is even more preferablethat average particle sizes of all of the non-magnetic powders satisfythe difference described above, with respect to the average particlesize of the ferromagnetic hexagonal ferrite powder.

From a viewpoint of controlling base friction, a mixing ratio of theferromagnetic hexagonal ferrite powder to the other non-magnetic powder(in a case of using two or more kinds of non-magnetic powders havingdifferent average particle sizes as other non-magnetic powder, the totalthereof), is preferably 90.0:10.0 (former:latter) to 99.9:0.1 and morepreferably 95.0:5.0 to 99.5:0.5 based on mass.

The magnetic layer described above can be provided on the surface of thenon-magnetic support directly or indirectly through the non-magneticlayer.

Non-Magnetic Layer

Next, the non-magnetic layer will be described.

The magnetic recording medium may include a magnetic layer directly on asurface of a non-magnetic support, or may include a non-magnetic layerincluding a non-magnetic powder and a binding agent between thenon-magnetic support and the magnetic layer. The non-magnetic powderincluded in the non-magnetic layer may be inorganic powder or organicpowder. In addition, carbon black and the like can be used. Examples ofthe inorganic powder include powder of metal, metal oxide, metalcarbonate, metal sulfate, metal nitride, metal carbide, and metalsulfide. These non-magnetic powder can be purchased as a commerciallyavailable product or can be manufactured by a well-known method. Fordetails thereof, descriptions disclosed in paragraphs 0036 to 0039 ofJP2010-24113A can be referred to. A content (filling percentage) of thenon-magnetic powder in the non-magnetic layer is preferably 50% to 90%by mass and more preferably 60% to 90% by mass.

In regards to other details of a binding agent or additives of thenon-magnetic layer, the well-known technology regarding the non-magneticlayer can be applied. In addition, in regards to the type and thecontent of the binding agent, and the type and the content of theadditive, for example, the well-known technology regarding the magneticlayer can be applied.

In the invention and the specification, the non-magnetic layer alsoincludes a substantially non-magnetic layer including a small amount offerromagnetic powder as impurities or intentionally, together with thenon-magnetic powder. Here, the substantially non-magnetic layer is alayer having a residual magnetic flux density equal to or smaller than10 mT, a layer having coercivity equal to or smaller than 7.96 kA/m(100Oe), or a layer having a residual magnetic flux density equal to orsmaller than 10 mT and coercivity equal to or smaller than 7.96 kA/m(100Oe). It is preferable that the non-magnetic layer does not have aresidual magnetic flux density and coercivity.

Non-Magnetic Support

Next, the non-magnetic support (hereinafter, also simply referred to asa “support”) will be described.

As the non-magnetic support, well-known components such as polyethyleneterephthalate, polyethylene naphthalate, polyamide, polyamide imide,aromatic polyamide subjected to biaxial stretching are used. Amongthese, polyethylene terephthalate, polyethylene naphthalate, andpolyamide are preferable. Corona discharge, plasma treatment,easy-bonding treatment, or heat treatment may be performed with respectto these supports in advance.

Back Coating Layer

The magnetic recording medium can also include a back coating layerincluding non-magnetic powder and a binding agent on a surface of thenon-magnetic support opposite to the surface provided with the magneticlayer. The back coating layer preferably includes any one or both ofcarbon black and inorganic powder. For the binding agent included in theback coating layer and various additives which can be randomly includedtherein, a well-known technology regarding the back coating layer can beapplied, and a well-known technology regarding the process of themagnetic layer and/or the non-magnetic layer can also be applied. Forexample, for the back coating layer, descriptions disclosed inparagraphs 0018 to 0020 of JP2006-331625A and page 4, line 65, to page5, line 38, of U.S. Pat. No. 7,029,774 can be referred to.

Various Thicknesses

Thicknesses of the non-magnetic support and each layer of the magneticrecording medium will be described below.

A thickness of the non-magnetic support is, for example, 3.0 to 80.0 μm,preferably 3.0 to 50.0 μm, and more preferably 3.0 to 10.0 μm.

A thickness of the magnetic layer can be optimized according to asaturation magnetization amount of a magnetic head used, a head gaplength, a recording signal band, and the like. The thickness of themagnetic layer is generally 10 nm to 100 nm, preferably 20 to 90 nm, andmore preferably 30 to 70 nm, from a viewpoint of realization ofhigh-density recording. The magnetic layer may be at least one layer, orthe magnetic layer can be separated to two or more layers havingdifferent magnetic properties, and a configuration regarding awell-known multilayered magnetic layer can be applied. A thickness ofthe magnetic layer which is separated into two or more layers is a totalthickness of the layers.

A thickness of the non-magnetic layer is, for example, equal to orgreater than 50 nm, preferably equal to or greater than 70 nm, and morepreferably equal to or greater than 100 nm. Meanwhile, the thickness ofthe non-magnetic layer is preferably equal to or smaller than 800 nm andmore preferably equal to or smaller than 500 nm.

A thickness of the back coating layer is preferably equal to or smallerthan 0.9 μm and more preferably 0.1 to 0.7 μm.

The thicknesses of various layers of the magnetic recording medium andthe non-magnetic support can be acquired by a well-known film thicknessmeasurement method. As an example, a cross section of the magneticrecording medium in a thickness direction is, for example, exposed by awell-known method of ion beams or microtome, and the exposed crosssection is observed with an electron microscope such as a scanningelectron microscope or a transmission electron microscope. In the crosssection observation, various thicknesses can be acquired as a thicknessacquired at one portion of the cross section in the thickness direction,or an arithmetical mean of thicknesses acquired at a plurality ofportions of two or more portions, for example, two portions which arerandomly extracted. In addition, the thickness of each layer may beacquired as a designed thickness calculated according to themanufacturing conditions.

Manufacturing Step

Preparation of Each Layer Forming Composition

Steps of preparing the composition for forming the magnetic layer, thenon-magnetic layer, or the back coating layer generally include at leasta kneading step, a dispersing step, and a mixing step provided beforeand after these steps, if necessary. Each step may be divided into twoor more stages. The components used in the preparation of each layerforming composition may be added at an initial stage or in a middlestage of each step. As the solvent, one kind or two or more kinds ofvarious solvents generally used for manufacturing a coating typemagnetic recording medium can be used. For the solvent, a descriptiondisclosed in a paragraph 0153 of JP2011-216149A can be referred to, forexample. In addition, each component may be separately added in two ormore steps. For example, the binding agent may be separately added inthe kneading step, the dispersing step, and a mixing step for adjustinga viscosity after the dispersion. In order to manufacture the magneticrecording medium, a well-known manufacturing technology of the relatedart can be used in various steps. In the kneading step, an open kneader,a continuous kneader, a pressure kneader, or a kneader having a strongkneading force such as an extruder is preferably used. For details ofthese kneading processes, descriptions disclosed in JP1989-106338A(JP-H01-106338A) and JP1989-79274A (JP-H01-79274A) can be referred to.As a disperser, a well-known disperser can be used. The filtering may beperformed by a well-known method in any stage for preparing each layerforming composition. The filtering can be performed by using a filter,for example. As the filter used in the filtering, a filter having a holediameter of 0.01 to 3 μm (for example, filter made of glass fiber orfilter made of polypropylene) can be used, for example.

As described above, in one aspect, regarding the control of the basefriction, a magnetic recording medium can be manufactured by using twoor more kinds of ferromagnetic hexagonal ferrite powders havingdifferent average particle sizes. That is, the magnetic layer can beformed with first ferromagnetic hexagonal ferrite powder, and one ormore kinds of ferromagnetic hexagonal ferrite powder having an averageparticle size greater than that of the first ferromagnetic hexagonalferrite powder, as ferromagnetic hexagonal ferrite powder. As preferredaspects of a forming method of such a magnetic layer, aspects of thefollowing (1) to (3) can be used. A combination of two or more aspectsdescribed below is a more preferred aspect of the forming method of themagnetic layer. The first ferromagnetic hexagonal ferrite powder is onekind of ferromagnetic hexagonal ferrite powder among the two or morekinds of ferromagnetic hexagonal ferrite powders and is preferablyferromagnetic hexagonal ferrite powder with the largest proportiondescribed above based on mass.

(1) An average particle size of the first ferromagnetic hexagonalferrite powder is 10 to 80 nm.

(2) A difference between an average particle size of the ferromagnetichexagonal ferrite powder having an average particle size greater thanthat of the first ferromagnetic hexagonal ferrite powder, and theaverage particle size of the first ferromagnetic hexagonal ferritepowder is 10 to 50 nm.

(3) A mixing ratio of the first ferromagnetic hexagonal ferrite powderto the ferromagnetic hexagonal ferrite powder having an average particlesize greater than that of the first ferromagnetic hexagonal ferritepowder is 90.0:10.0 (former:latter) to 99.9:0.1 based on mass.

In another aspect, a magnetic recording medium can also be manufacturedby using non-magnetic powder other than the oxide abrasive and theprojection formation agent, as the non-magnetic powder of the magneticlayer. That is, the magnetic layer can be formed with the othernon-magnetic powder. As preferred aspects of a forming method of such amagnetic layer, aspects of the following (4) to (6) can be used. Acombination of two or more aspects described below is a more preferredaspect of the forming method of a magnetic layer.

(4) An average particle size of the other non-magnetic powder is greaterthan an average particle size of the ferromagnetic hexagonal ferritepowder.

(5) A difference between the average particle size of the ferromagnetichexagonal ferrite powder and the average particle size of the othernon-magnetic powder is 10 to 80 nm.

(6) A mixing ratio of the ferromagnetic hexagonal ferrite powder to theother non-magnetic powder is 90.0:10.0 (former:latter) to 99.9:0.1 basedon mass.

The value of the FIB abrasive diameter tends to decrease, as the oxideabrasive is present in a finer state in the magnetic layer. As means forcausing the oxide abrasive to be present in a finer state in themagnetic layer, a dispersing agent capable of increasing dispersibilityof the oxide abrasive can be used, as described above. In addition, inorder to cause the oxide abrasive to be present in a finer state in themagnetic layer, it is preferable that an abrasive having a smallparticle size is used, aggregation of the abrasive is prevented, anduneven distribution is prevented to disperse the abrasive in themagnetic layer evenly. As means for this, a method of reinforcingdispersion conditions of the oxide abrasive in a case of preparing themagnetic layer forming composition is used. For example, dispersing theoxide abrasive separately from the ferromagnetic hexagonal ferritepowder is one aspect of the reinforcement of the dispersion conditions.The separate dispersion is specifically a method of preparing a magneticlayer forming composition through a step of mixing an abrasive solutionincluding an oxide abrasive and a solvent (here, ferromagnetic hexagonalferrite powder is not substantially included) with a magnetic liquidincluding the ferromagnetic hexagonal ferrite powder, a solvent, and abinding agent. By performing the mixing after dispersing the oxideabrasive separately from the ferromagnetic hexagonal ferrite powder, itis possible to increase dispersibility of the oxide abrasive in themagnetic layer forming composition. The expression “ferromagnetic powderis not substantially included” means that the ferromagnetic hexagonalferrite powder is not added as a constituent element of the abrasivesolution, and a small amount of the ferromagnetic hexagonal ferritepowder mixed as impurities without any intention is allowed. In additionto the separate dispersion or with the separate dispersion, means suchas the dispersion process performed for a long period of time, the useof dispersion medium having a small size (for example, a decrease indiameter of dispersion beads in the beads dispersion), a high degree offilling of the dispersion medium in the disperser can be randomlycombined to reinforce the dispersion conditions. For the disperser andthe dispersion medium, a commercially available product can be used. Inaddition, a centrifugal separation process of the abrasive solutioncontributes to the oxide abrasive present in the magnetic layer in afiner state, by removing particles having a size greater than an averageparticle size and/or aggregated particles from the particles configuringthe oxide abrasive. The centrifugal separation process can be performedby a commercially available centrifugal separator. In addition, thefiltering of the abrasive solution performed by using a filter or thelike is preferable for removing a coarse aggregate of the aggregatedparticles configuring the oxide abrasive. The removal of such coarseaggregate can contribute to the oxide abrasive present in the magneticlayer in a finer state. For example, the filtering by using a filterhaving a smaller hole diameter can contribute to the oxide abrasivepresent in the magnetic layer in a finer state. In addition, byadjusting various process conditions (for example, stirring conditions,dispersion process conditions, filtering conditions, and the like) aftermixing the abrasive solution with the component for preparing themagnetic layer forming composition such as the ferromagnetic hexagonalferrite powder or the like, it is possible to increase dispersibility ofthe oxide abrasive in the magnetic layer forming composition. This canalso contribute to the oxide abrasive present in the magnetic layer in afiner state. However, in a case where the oxide abrasive is present inthe magnetic layer in an extremely finer state, the FIB abrasivediameter may be smaller than 0.04 μm, and therefore, it is preferablethat various conditions for preparing the abrasive solution are adjustedso as to realize the FIB abrasive diameter of 0.04 μm to 0.08 μm.

Regarding the dispersion process of the magnetic layer formingcomposition, as described above, it is preferable to prevent theoccurrence of chipping. In order for this, it is preferable to performthe dispersion process of the ferromagnetic hexagonal ferrite powder bya dispersion process having two stages, in which a coarse aggregate ofthe ferromagnetic hexagonal ferrite powder is crushed by the dispersionprocess in a first stage, and the dispersion process in a second stage,in which a collision energy applied to particles of the ferromagnetichexagonal ferrite powder due to collision with the dispersion beads issmaller than that in the first dispersion process, is performed, in thestep of preparing the magnetic layer forming composition. According tosuch a dispersion process, it is possible to improve dispersibility ofthe ferromagnetic hexagonal ferrite powder and prevent the occurrence ofchipping.

As a preferred aspect of the dispersion process having two stages, adispersion process including a first stage of obtaining a dispersionliquid by performing the dispersion process of the ferromagnetichexagonal ferrite powder, the binding agent, and the solvent under thepresence of first dispersion beads, and a second stage of performing thedispersion process of the dispersion liquid obtained in the first stageunder the presence of second dispersion beads having smaller beaddiameter and density than those of the first dispersion beads can beused. Hereinafter, the dispersion process of the preferred aspectdescribed above will be further described.

In order to increase the dispersibility of the ferromagnetic hexagonalferrite powder, the first stage and the second stage are preferablyperformed as the dispersion process before mixing the ferromagnetichexagonal ferrite powder and other powder components. For example, thefirst stage and the second stage are preferably performed as adispersion process of a solution (magnetic liquid) includingferromagnetic hexagonal ferrite powder, a binding agent, a solvent, andrandomly added additives, before mixing the oxide abrasive (preferablybefore mixing the oxide abrasive and the projection formation agentdescribed above).

A bead diameter of the second dispersion bead is preferably equal to orsmaller than 1/100 and more preferably equal to or smaller than 1/500 ofa bead diameter of the first dispersion bead. The bead diameter of thesecond dispersion bead can be, for example, equal to or greater than1/10,000 of the bead diameter of the first dispersion bead. However,there is no limitation to this range. The bead diameter of the seconddispersion bead is, for example, preferably 80 to 1,000 nm. Meanwhile,the bead diameter of the first dispersion bead can be, for example, 0.2to 1.0 mm.

The bead diameter of the invention and the specification is a valuemeasured by the same method as the measurement method of the averageparticle size of the powder described above.

The second stage is preferably performed under the conditions in whichthe amount of the second dispersion beads is equal to or greater than 10times of the amount of the ferromagnetic hexagonal ferrite powder, andis more preferably performed under the conditions in which the amount ofthe second dispersion beads is 10 times to 30 times of the amount of theferromagnetic hexagonal ferrite powder, based on mass.

Meanwhile, the amount of the dispersion beads in the first stage ispreferably in the range described above.

The second dispersion beads are beads having lower density than that ofthe first dispersion beads. The “density” is obtained by dividing themass (unit: g) of the dispersion beads by volume (unit: cm³). Themeasurement is performed by the Archimedes method. The density of thesecond dispersion beads is preferably equal to or lower than 3.7 g/cm³and more preferably equal to or lower than 3.5 g/cm³. The density of thesecond dispersion beads may be, for example, equal to or higher than 2.0g/cm³ or may be lower than 2.0 g/cm³. As the preferred second dispersionbeads from a viewpoint of density, diamond beads, silicon carbide beads,or silicon nitride beads can be used, and as preferred second dispersionbeads from a viewpoint of density and hardness, diamond beads can beused.

Meanwhile, as the first dispersion beads, dispersion beads havingdensity exceeding 3.7 g/cm³ are preferable, dispersion beads havingdensity equal to or higher than 3.8 g/cm³ are more preferable, anddispersion beads having density equal to or higher than 4.0 g/cm³ areeven more preferable. The density of the first dispersion beads may be,for example, equal to or smaller than 7.0 g/cm³ or may exceed 7.0 g/cm³.As the first dispersion beads, zirconia beads or alumina beads arepreferably used, and zirconia beads are more preferably used.

The dispersion time is not particularly limited and may be set inaccordance with the kind of a disperser used.

Coating Step

The magnetic layer can be formed by directly applying the magnetic layerforming composition onto the surface of the non-magnetic support orperforming multilayer coating of the magnetic layer forming compositionwith the non-magnetic layer forming composition in order or at the sametime. The back coating layer can be formed by applying the back coatinglayer forming composition to the surface side of the non-magneticsupport opposite to a surface side provided with the magnetic layer (orto be provided with the magnetic layer). For details of the coating forforming each layer, a description disclosed in a paragraph 0066 ofJP2010-231843A can be referred to.

Other Steps

For details of various other steps for manufacturing the magneticrecording medium, descriptions disclosed in paragraphs 0067 to 0070 ofJP2010-231843A can be referred to. It is preferable that the coatinglayer of the magnetic layer forming composition is subjected to analignment process, while the coating layer is wet (not dried). For thealignment process, various well-known technologies such as a descriptiondisclosed in a paragraph 0067 of JP2010-231843A can be used. Asdescribed above, it is preferable to perform the homeotropic alignmentprocess as the alignment process, from a viewpoint of controlling theXRD intensity ratio. Regarding the alignment process, the abovedescription can also be referred to.

As described above, it is possible to obtain the magnetic recordingmedium according to one aspect of the invention. However, themanufacturing method described above is merely an example, values of theXRD intensity ratio, the vertical squareness ratio, the base friction,and the FIB abrasive diameter can be controlled to be in respectiveranges described above by any means capable of adjusting the valuesthereof, and such an aspect is also included in the invention.

The magnetic recording medium according to one aspect of the inventioncan be a tape-shaped magnetic recording medium (magnetic tape) or canalso be a disk-shaped magnetic recording medium (magnetic disk). Forexample, the magnetic tape is normally accommodated in a magnetic tapecartridge and the magnetic tape cartridge is mounted in a magneticrecording and reproducing device. A servo pattern can also be formed inthe magnetic tape by a well-known method, in order to allow headtracking servo to be performed in the magnetic recording and reproducingdevice.

Magnetic Recording and Reproducing Device

One aspect of the invention relates to a magnetic recording andreproducing device including the magnetic recording medium and amagnetic head.

In the invention and the specification, the “magnetic recording andreproducing device” means a device capable of performing at least one ofthe recording of information on the magnetic recording medium or thereproducing of information recorded on the magnetic recording medium.Such a device is generally called a drive. The magnetic head included inthe magnetic recording and reproducing device can be a recording headcapable of performing the recording of information on the magneticrecording medium, or can be a reproducing head capable of performing thereproducing of information recorded on the magnetic recording medium. Inaddition, in one aspect, the magnetic recording and reproducing devicecan include both of a recording head and a reproducing head as separatemagnetic heads. In another aspect, the magnetic head included in themagnetic recording and reproducing device can also have a configurationof including both of a recording element and a reproducing element inone magnetic head. In addition, the magnetic head which performs therecording of information and/or the reproducing of information mayinclude a servo pattern reading element. Alternatively, as a head otherthan the magnetic head which performs the recording of informationand/or the reproducing of information, a magnetic head (servo head)including a servo pattern reading element may be included in themagnetic recording and reproducing device.

In the magnetic recording and reproducing device, the recording ofinformation on the magnetic recording medium and the reproducing ofinformation recorded on the magnetic recording medium can be performedby bringing the surface of the magnetic layer of the magnetic recordingmedium into contact with the magnetic head and sliding. The magneticrecording and reproducing device may include the magnetic recordingmedium according to one aspect of the invention and well-knowntechnologies can be applied for other configurations.

The magnetic recording medium according to one aspect of the inventioncan exhibit excellent electromagnetic conversion characteristics in themagnetic recording and reproducing device. That is, in the magneticrecording and reproducing device, the information recorded on themagnetic recording medium according to one aspect of the invention canbe reproduced at a high SNR. In addition, in the magnetic recording andreproducing device, the GTT can be performed while changing the magneticrecording medium with a new product. In this GTT, according to themagnetic recording medium according to one aspect of the invention, itis possible to prevent occurrence of the head element chipping. Theelement for preventing the occurrence of the head element chipping canbe one or more elements selected from the group consisting of areproducing element, recording element, and a servo pattern readingelement, and two or more elements can also be used. The reproducingelement is preferably a magnetoresistive (MR) element capable of readinginformation recorded on the magnetic recording medium with excellentsensitivity. In addition, the MR element is also preferable as the servopattern reading element. As a head (MR head) including the MR element asthe reproducing element and/or the servo pattern reading element,various well-known MR heads can be used.

EXAMPLES

Hereinafter, the invention will be described with reference to examples.However, the invention is not limited to aspects shown in the examples.“Parts” and “%” in the following description mean “parts by mass” and “%by mass”, unless otherwise noted. In addition, steps and evaluationsdescribed below are performed in an environment of an atmospheretemperature of 23° C.±1° C., unless otherwise noted. Further, “eq”described below is an equivalent which is a unit which cannot beconverted into the SI unit system.

Example 1

A list of each layer forming composition is shown below.

Preparation of Abrasive Solution

The amount of 2,3-dihydroxynaphthalene (manufactured by Tokyo ChemicalIndustry Co., Ltd.) shown in condition C of Table 1, 31.3 parts of a 32%solution (solvent is a mixed solvent of methyl ethyl ketone and toluene)of a polyester polyurethane resin having a SO₃Na group as a polar group(UR-4800 (amount of a polar group: 80 meq/kg) manufactured by ToyoboCo., Ltd.), and 570.0 parts of a mixed solution of methyl ethyl ketoneand cyclohexanone (mass ratio of 1:1) as a solvent were mixed in 100.0parts of an oxide abrasive (alumina powder) shown in condition C ofTable 1, and dispersed in the presence of zirconia beads (bead diameter:0.1 mm) by a paint shaker for a period of time shown in condition C ofTable 1 (bead dispersion time). After the dispersion, the centrifugalseparation process of a dispersion liquid obtained by separating thedispersion liquid from the beads by mesh was performed. The centrifugalseparation process was performed by using CS150GXL manufactured byHitachi, Ltd. (rotor used is S100AT6 manufactured by Hitachi, Ltd.) as acentrifugal separator at a rotation per minute (rpm) shown in thecondition C of Table 1, for a period of time (centrifugal separationtime) shown in the condition C of Table 1. After that, the filtering wasperformed by using a filter having a hole diameter shown in thecondition C of Table 1, and an alumina dispersion (abrasive solution)was obtained.

Preparation of Magnetic Layer Forming Composition

Magnetic Liquid

Plate-shaped ferromagnetic hexagonal ferrite powder (M-type bariumferrite): 100.0 parts

Two kinds of ferromagnetic hexagonal ferrite powders below are used

-   -   Ferromagnetic hexagonal ferrite powder (1)    -   Average particle size and amount used: see Table 2    -   Ferromagnetic hexagonal ferrite powder (2)    -   Average particle size and amount used: see Table 2

-   Oleic acid: 2.0 parts

-   A vinyl chloride copolymer (MR-104 manufactured by Zeon    Corporation): 10.0 parts

-   SO₃Na group-containing polyurethane resin: 4.0 parts    -   (Weight-average molecular weight: 70,000, SO₃Na group: 0.07        meq/g)

-   An amine-based polymer (DISPERBYK-102 manufactured by BYK Additives    & Instruments): 6.0 parts

-   Methyl ethyl ketone: 150.0 parts

-   Cyclohexanone: 150.0 parts

-   Abrasive Solution

-   Abrasive solution described above: 6.0 parts

-   Projection Formation Agent Liquid (Silica Sol)

-   Colloidal silica: 2.0 parts    -   (Average particle size: 80 nm)

-   Methyl ethyl ketone: 8.0 parts

-   Other components

-   Stearic acid: 3.0 parts

-   Stearic acid amide: 0.3 parts

-   Butyl stearate: 6.0 parts

-   Methyl ethyl ketone: 110.0 parts

-   Cyclohexanone: 110.0 parts

-   Polyisocyanate (CORONATE (registered trademark) L manufactured by    Tosoh Corporation): 3.0 parts

Preparation Method

A dispersion liquid A was prepared by dispersing (first stage) variouscomponents of the magnetic liquid with a batch type vertical sand millby using zirconia beads having a bead diameter of 0.5 mm (firstdispersion beads, density of 6.0 g/cm³) for 24 hours, and thenperforming filtering with a filter having a hole diameter of 0.5 μm. Theused amount of zirconia beads was 10 times of the amount of theferromagnetic hexagonal ferrite powder based on mass.

After that, a dispersion liquid (dispersion liquid B) was prepared bydispersing (second stage) dispersion liquid A with a batch type verticalsand mill by using diamond beads having a bead diameter shown in Table 2(second dispersion beads, density of 3.5 g/cm³) for a period of timeshown in Table 2, and then separating diamond beads by using acentrifugal separator. The magnetic liquid is the dispersion liquid Bobtained as described above.

The magnetic liquid, the abrasive solution, the projection formationagent liquid, and the other components were introduced in a dissolverstirrer and stirred at a circumferential speed of 10 m/sec for a periodof time shown in the condition C of Table 1 (stirring time). After that,a ultrasonic dispersion process was performed at a flow rate of 7.5kg/min with a flow type ultrasonic disperser for a period of time shownin the condition C of Table 1 (ultrasonic dispersion time), andfiltering with a filter having a hole diameter shown in the condition Cof Table 1 was performed for the number of times shown in the conditionC of Table 1, thereby preparing the magnetic layer forming composition.

Preparation of Non-Magnetic Layer Forming Composition

A non-magnetic layer forming composition was prepared by dispersingvarious components of the non-magnetic layer forming composition with abatch type vertical sand mill by using zirconia beads having a beaddiameter of 0.1 mm for 24 hours, and then performing filtering with afilter having a hole diameter of 0.5 μm.

Non-magnetic inorganic powder: α-iron oxide: 100.0 parts

-   -   (Average particle size: 10 nm, BET specific surface area: 75        m²/g)

-   Carbon black: 25.0 parts    -   (Average particle size: 20 nm)

-   A SO₃Na group-containing polyurethane resin: 18.0 parts    -   (Weight-average molecular weight: 70,000, content of SO₃Na        group: 0.2 meq/g)

-   Stearic acid: 1.0 parts

-   Cyclohexanone: 300.0 parts

-   Methyl ethyl ketone: 300.0 parts

Preparation of Back Coating Layer Forming Composition

Components among various components of the back coating layer formingcomposition except a lubricant (stearic acid and butyl stearate),polyisocyanate, and 200.0 parts of cyclohexanone were kneaded anddiluted by an open kneader, and subjected to a dispersion process of 12passes, with a transverse beads mill disperser and zirconia beads havinga bead diameter of 1 mm, by setting a bead filling percentage as 80volume %, a circumferential speed of rotor distal end as 10 m/sec, and aretention time for 1 pass as 2 minutes. After that, the remainingcomponents were added and stirred with a dissolver, the obtaineddispersion liquid was filtered with a filter having a hole diameter of 1μm and a back coating layer forming composition was prepared.

Non-magnetic inorganic powder: α-iron oxide: 80.0 parts

-   -   (Average particle size: 0.15 μm, BET specific surface area: 52        m²/g)

-   Carbon black: 20.0 parts    -   (Average particle size: 20 nm)

-   A vinyl chloride copolymer: 13.0 parts

-   A sulfonic acid salt group-containing polyurethane resin: 6.0 parts

-   Phenylphosphonic acid: 3.0 parts

-   Cyclohexanone: 155.0 parts

-   Methyl ethyl ketone: 155.0 parts

-   Stearic acid: 3.0 parts

-   Butyl stearate: 3.0 parts

-   Polyisocyanate: 5.0 parts

-   Cyclohexanone: 200.0 parts

Manufacturing of Magnetic Tape

The non-magnetic layer forming composition prepared as described abovewas applied to a surface of a support made of polyethylene naphthalatehaving a thickness of 5.0 μm so that the thickness after the dryingbecomes 100 nm and was dried to form a non-magnetic layer. The magneticlayer forming composition prepared as described above was applied ontothe surface of the formed non-magnetic layer so that the thickness afterthe drying becomes 70 μm and a coating layer was formed. A homeotropicalignment process was performed by applying a magnetic field having astrength shown in Table 2 in a vertical direction with respect to thesurface of the coating layer, while the coating layer of the magneticlayer forming composition is wet (not dried). After that, the coatinglayer was dried.

After that, the back coating layer forming composition prepared asdescribed above was applied to the surface of the support opposite tothe surface where the non-magnetic layer and the magnetic layer wereformed, so that the thickness after the drying becomes 0.4 μm, and wasdried. A calender process (surface smoothing treatment) was performedwith respect to the tape obtained as described above by a calenderconfigured of only a metal roll, at a speed of 100 m/min, linearpressure of 300 kg/cm (294 kN/m), and by using a calender roll at asurface temperature of 90° C., and then, a heat treatment was performedin the environment of the atmosphere temperature of 70° C. for 36 hours.After the heat treatment, the slitting was performed to have a width of½ inches (0.0127 meters), and a servo pattern was formed on the magneticlayer by a commercially available servo writer.

By doing so, a magnetic tape of Example 1 was obtained.

Examples 2 to 9 and Comparative Examples 1 to 13

A magnetic tape was manufactured in the same manner as in Example 1,except that various items shown in Table 1 and Table 2 were changed asshown in each table.

All of the oxide abrasive shown in Table 1 are alumina powder.

In Table 2, in the comparative examples in which “none” is shown in acolumn of the dispersion beads and a column of the time, the magneticlayer forming composition was prepared without performing the secondstage in the magnetic liquid dispersion process.

In Table 2, in the examples in which “none” is shown in a column of thehomeotropic alignment process magnetic field strength, the magneticlayer was formed without performing the alignment process.

The amount of the ferromagnetic hexagonal ferrite powder shown in Table2 is content of each ferromagnetic hexagonal ferrite powder based onmass with respect to 100.0 parts by mass of a total of the ferromagnetichexagonal ferrite powder. An average particle size of the ferromagnetichexagonal ferrite powder shown in Table 2 is a value obtained bycollecting the necessary amount from a batch of the powder used in thepreparation of the magnetic tape and measuring an average particle sizeby the method described above. The ferromagnetic hexagonal ferritepowder after measuring the average particle size was used in thepreparation of a magnetic liquid for preparing the magnetic tape.

Evaluation of Physical Properties of Magnetic Tape

(1) XRD Intensity Ratio

A tape sample was cut out from the manufactured magnetic tape.

Regarding the cut-out tape sample, the surface of the magnetic layer wasirradiated with X-ray by using a thin film X-ray diffraction device(Smart Lab manufactured by Rigaku Corporation), and the In-Plane XRD wasperformed by the method described above.

The peak intensity Int(114) of the diffraction peak of the (114) planeand the peak intensity Int(110) of the diffraction peak of a (110) planeof a hexagonal ferrite crystal structure were obtained from the X-raydiffraction spectra obtained by the In-Plane XRD, and the XRD intensityratio (Int(110)/Int(114)) was calculated.

(2) Vertical Squareness Ratio

A vertical squareness ratio of each manufactured magnetic tape wasobtained by the method described above using a vibrating samplemagnetometer (manufactured by Toei Industry Co., Ltd.).

(3) Base Friction

First, marking was performed on a measurement surface with a lasermarker in advance, and an atomic force microscope (AFM) image of aportion separated from the mark by a certain distance (approximately 100μm) was observed. The observation was performed regarding an area of avisual field of 7 μm×7 μm. At this time, marking was performed on theARM by changing a cantilever to a hard material (single crystalsilicon), so as to easily capture a scanning electron microscope (SEM)image of the same portion as will be described later. All of projectionshaving a height equal to or greater than 15 nm from the referencesurface were extracted from the AFM image observed as described above. Aportion in which it is determined that projections were not present, wasspecified as a base portion, and the base friction was measured withTI-950 type TriboIndenter manufactured by Hysitron, Inc. by the methoddescribed above.

An SEM image of the same portion as the portion observed with the AFMimage was observed to obtain a component map, and it was confirmed thatthe extracted projections having a height equal to or greater than 15 nmfrom the reference surface were projections formed of alumina orcolloidal silica. In the examples and the comparative examples, in thecomponent map obtained with the SEM, alumina and colloidal silica werenot confirmed in the base portion. Here, the component analysis wasperformed with the SEM, but the component analysis is not limited tobeing performed with the SEM, and can be performed by a well-knownmethod such as energy dispersive X-ray spectrometry (EDS) or augerelectron spectroscopy (AES).

(4) FIB Abrasive Diameter

The FIB abrasive diameter of each manufactured magnetic tape wasobtained by the following method.

As a focused ion beam device, MI4050 manufactured by HitachiHigh-Technologies Corporation was used, and the image analysis software,ImageJ which is free software was used.

(i) Acquiring of Secondary Ion Image

The surface of the back coating layer of the sample for measurement cutout from each manufactured magnetic tape was bonded to an adhesive layerof a commercially available carbon double-sided tape for SEM measurement(double-sided tape in which a carbon film is formed on a base materialformed of aluminum). An adhesive layer of this double-sided tape on asurface opposite to the surface bonded to the surface of the backcoating layer was bonded to a sample table of the focused ion beamdevice. By doing so, the sample for measurement was disposed on thesample table of the focused ion beam device so that the surface of themagnetic layer faces upwards.

Without performing the coating process before the imaging, the beamsetting of the focused ion beam device was set so that an accelerationvoltage is 30 kV, a current value is 133 pA, a beam size is 30 nm, and abrightness is 50%, and an SI signal was detected by a secondary iondetector. ACB was carried out at three spots on a non-imaged region ofthe surface of the magnetic layer to stabilize a color of the image.Then, the contrast reference value and the brightness reference valuewere determined. The brightness reference value as determined in theabove ACB and the contrast value which was lowered by 1% from thecontrast reference value as determined in the above ACB were determinedas the conditions for capturing a secondary ion image. A non-imagedregion of the surface of the magnetic layer was selected and imaging wasperformed under the conditions for capturing as determined above and ata pixel distance of 25.0 (nm/pixel). As an image capturing method,PhotoScan Dot×4_Dwell Time 15 μsec (capturing time: 1 min), and acapturing size was set as 25 μm×25 μm. By doing so, a secondary ionimage of a region of the surface of the magnetic layer having a size of25 μm×25 μm was obtained. After the scanning, the obtained secondary ionimage was stored as a file format, JPEG, by ExportImage, by clickingmouse right button on the captured screen. The pixel number of the imagewhich was 2,000 pixel×2,100 pixel was confirmed, the cross mark and themicron bar on the captured image were deleted, and an image of 2,000pixel×2,000 pixel was obtained.

(ii) Calculation of FIB Abrasive Diameter

The image data of the secondary ion image obtained in (i) was draggedand dropped in ImageJ which is the image analysis software.

A tone of the image data was changed to 8 bit by using the imageanalysis software. Specifically, Image of the operation menu of theimage analysis software was clicked and 8 bit of Type was selected.

For the binarization process, 250 gradations was selected as a lowerlimit value, 255 gradations was selected as an upper limit value, andthe binarization process was executed by these two threshold values.Specifically, on the operation menu of the image analysis software,Image was clicked, Threshold of Adjust was selected, the lower limitvalue was selected as 250, the upper limit value was selected as 255,and then, apply was selected. Regarding the obtained image, Process ofthe operation menu of the image analysis software was clicked, Despeckleof Noise was selected, and Size 4.0-Infinity was set on AnalyzeParticleto remove noise components.

Regarding the binarization process image obtained as described above,AnalyzeParticle was selected from the operation menu of the imageanalysis software, and the number and Area (unit: Pixel) ofwhite-shining portions on the image were obtained. The area of eachwhite-shining portion on the image was obtained by converting Area(unit: Pixel) into the area by the image analysis software.Specifically, 1 pixel of the image obtained under the imaging conditionscorresponded to 0.0125 μm, and accordingly, the area A [μm²] wascalculated by an expression, area A=Area pixel×0.0125{circumflex over( )}2. By using the area calculated as described above, an equivalentcircle diameter L of each white-shining portion was obtained by anexpression, equivalent circle diameter L=(A/π){circumflex over( )}(½)×2=L.

The above step was performed four times at different portions (25 μm×25μm) of the surface of the magnetic layer of the sample for measurement,and the FIB abrasive diameter was calculated from the obtained result byan expression, FIB abrasive diameter=Σ(Li)/Σi.

Evaluation of Electromagnetic Conversion Characteristics (SNR)

The electromagnetic conversion characteristics of each manufacturedmagnetic tape were measured with a reel tester having a width of ½inches (0.0127 meters) to which a head was fixed, by the followingmethod. The following recording and reproducing were performed bysliding the surface of the magnetic layer of the magnetic tape and thehead.

A running speed of the magnetic tape (magnetic head/magnetic taperelative speed) was set as 4 m/sec. As a recording head, a metal-in-gap(MIG) head (gap length of 0.15 μm, track width of 1.0 μm) was used, anda recording current was set as an optimal recording current of eachmagnetic tape. As a reproducing head, a giant-magnetoresistive (GMR)head having an element thickness of 15 nm, a shield interval of 0.1 μm,and a lead width of 0.5 μm was used. A signal was recorded at linearrecording density (300 kfci) and a reproducing signal was measured witha spectrum analyzer manufactured by Shibasoku Co., Ltd. A ratio of anoutput value of a carrier signal and integral noise over whole spectralrange was set as an SNR. For the SNR measurement, a part of a signalwhich is sufficiently stabilized after running of the magnetic tape wasused. The SNR was shown in Table 2 as a relative value in a case wherethe SNR of Comparative Example 1 was set as 0.0 dB. The unit kfci is aunit of linear recording density (cannot be converted into the unit SI).

Head Element Chipping Amount in GTT

A magnetic head (MR head) used in a tape drive of TS 1140 manufacturedby IBM was attached to a reel tester, the magnetic tape having a lengthof 1000 m of 1 reel was caused to run for 200 passes, by setting arunning speed (magnetic head/magnetic tape relative speed) as 4 m/secwhile sliding the surface of the magnetic layer and the MR head.

The same running of 200 passes was repeated by replacing the magnetictape with a new product (1,000 reels of the magnetic tape were used),and the chipping amount of the MR element of the MR head was measured bythe following method.

A carbon film was vapor-deposited on the surface of the MR headincluding a surface slid on the surface magnetic layer by using a vacuumdeposition device (JEE-4X manufactured by JEOL), and a platinum film wasformed by sputtering on this carbon film by using an ion sputteringdevice (E-1030 manufactured by Hitachi High-Technologies Corporation.After that, a cross section parallel to the sliding direction of themagnetic tape in a case of running, was exposed from the MR head byusing A FIB-SEM combined machine, Helios 400S manufactured by MRFEI, anda sample for cross section observation (piece having a thickness of 100nm) was cut out so that the MR element was included in the crosssection. A distance in a vertical direction between the sliding surfaceof the surface of the magnetic layer and the top of the MR element wasobtained by using a TEM image obtained by observing the sample for crosssection observation using a transmission electron microscope (TEM)(Titan 80-300 manufactured by FBI) at an acceleration voltage of 300 kV.A difference between the obtained distance and a distance obtained bythe same method regarding unused MR head was shown as the head elementchipping amount in GTT in Table 2.

TABLE 1 Conditions A B C D E F Preparation of Oxide abrasive productname Hit80 Hit80 Hit80 Hit100 Hit70 Hit80 abrasive solution(manufactured by Sumitomo Chemical Co., Ltd.) Oxide abrasive BETspecific surface 30 30 30 40 20 30 area (m²/g) Content of abrasivesolution dispersing 3.0 parts 0 part 3.0 parts 3.0 parts 3.0 parts 3.0parts agent (2,3-dihydroxynaphthalene) Beads dispersion time 5 min 60min 60 min 180 min 60 min 180 min Centrifugal separation Rotation rateNone 3500 rpm 3500 rpm 3500 rpm 5500 rpm 3500 rpm Centrifugal None 4 min4 min 4 min 4 min 4 min separation time Filter hole diameter 0.5 μm 0.3μm 0.3 μm 0.3 μm 0.3 μm 0.3 μm Preparation of Stirring time 30 min 60min 360 min 360 min 180 min 360 min magnetic layer Ultrasonic dispersiontime 0.5 min 60 min 60 min 60 min 60 min 60 min forming compositionFilter hole diameter 0.5 μm 0.3 μm 0.3 μm 0.3 μm 0.3 μm 0.3 μm Number oftimes of filter process  1  2  3  3  2  3

TABLE 2 Magnetic liquid dispersion process second stage Dispersion beadsFerromagnetic hexagonal Ferromagnetic hexagonal Used amount (mass ofbeads Homeotropic ferrite powder (1) ferrite powder (2) with respect tomass of alignment process Average Amount Average Amount Beadferromagnetic hexagonal magnetic field particle size used particle sizeused Kind diameter ferrite powders (1) and (2)) Time strengthComparative 22 nm  100% — — None None None None None Example 1Comparative 22 nm  100% — — None None None None None Example 2Comparative 22 nm  100% — — None None None None None Example 3Comparative 22 nm  100% — — None None None None None Example 4Comparative 22 nm  100% — — Diamond 500 nm 10 times 1 h 0.15T Example 5Comparative 22 nm  100% — — Diamond 500 nm 10 times 1 h 0.15T Example 6Comparative 22 nm  100% — — Diamond 500 nm 10 times 1 h 0.15T Example 7Comparative 22 nm  100% — — Diamond 500 nm 10 times 1 h 0.15T Example 8Comparative 22 nm 99.0% 60 nm 1.0% None None None None None Example 9Comparative 22 nm 99.0% 60 nm 1.0% None None None None 0.15T Example 10Comparative 22 nm 99.0% 60 nm 1.0% None None None None 0.30T Example 11Comparative 22 nm 99.0% 60 nm 1.0% Diamond 500 nm 10 times 1 h 1.00TExample 12 Comparative 22 nm 99.0% 60 nm 1.0% Diamond 500 nm 10 times 1h None Example 13 Example 1 22 nm 99.0% 60 nm 1.0% Diamond 500 nm 10times 1 h 0.15T Example 2 22 nm 99.0% 60 nm 1.0% Diamond 500 nm 10 times1 h 0.20T Example 3 22 nm 99.0% 60 nm 1.0% Diamond 500 nm 10 times 1 h0.30T Example 4 22 nm 99.0% 60 nm 1.0% Diamond 500 nm 10 times 1 h 0.50TExample 5 22 nm 99.0% 60 nm 1.0% Diamond 500 nm 20 times 1 h 0.15TExample 6 22 nm 99.0% 60 nm 1.0% Diamond 500 nm 10 times 1 h 0.30TExample 7 22 nm 99.0% 60 nm 1.0% Diamond 500 nm 10 times 1 h 0.30TExample 8 22 nm 98.8% 60 nm 1.2% Diamond 500 nm 10 times 1 h 0.30TExample 9 22 nm 98.5% 60 nm 1.5% Diamond 500 nm 10 times 1 h 0.30TPreparation condition of abrasive solution and Head element magneticlayer chipping forming Base XRD intensity ratio Vertical FIB abrasiveSNR amount in GTT composition friction Int(110)/Int(114) squarenessratio diameter (dB) (nm) Comparative A 0.45 0.2 0.55 0.16 μm 0.0 10.0Example 1 Comparative B 0.45 0.2 0.55 0.11 μm 1.1 7.0 Example 2Comparative C 0.45 0.2 0.55 0.06 μm 2.0 4.1 Example 3 Comparative D 0.450.2 0.55 0.03 μm 2.9 5.9 Example 4 Comparative C 0.45 0.5 0.70 0.06 μm5.1 4.0 Example 5 Comparative A 0.30 0.5 0.70 0.16 μm 3.0 9.1 Example 6Comparative B 0.30 0.5 0.70 0.11 μm 4.0 6.0 Example 7 Comparative D 0.300.5 0.70 0.03 μm 5.9 7.9 Example 8 Comparative C 0.30 0.3 0.56 0.06 μm2.1 0.0 Example 9 Comparative C 0.30 3.8 0.62 0.06 μm 2.0 0.0 Example 10Comparative C 0.30 5.0 0.76 0.06 μm 2.1 0.0 Example 11 Comparative C0.30 6.2 0.88 0.06 μm 2.0 0.0 Example 12 Comparative C 0.30 0.3 0.650.06 μm 2.0 0.0 Example 13 Example 1 C 0.30 0.5 0.69 0.06 μm 5.1 0.0Example 2 C 0.30 1.5 0.75 0.06 μm 5.0 0.0 Example 3 C 0.30 2.4 0.81 0.06μm 5.1 0.0 Example 4 C 0.30 4.0 0.85 0.06 μm 5.0 0.0 Example 5 C 0.300.7 0.83 0.06 μm 5.1 0.0 Example 6 E 0.30 2.4 0.81 0.08 μm 5.5 0.0Example 7 F 0.30 2.4 0.81 0.04 μm 5.5 0.0 Example 8 C 0.25 2.4 0.81 0.06μm 5.0 0.0 Example 9 C 0.22 2.4 0.81 0.06 μm 5.0 0.0

From the results shown in Table 2, it is possible to confirm that, inExamples 1 to 9 in which the XRD intensity ratio of the magnetic tape,the vertical squareness ratio, the base friction, and the FIB abrasivediameter are in the ranges described above, the reproduction can beperformed at a high SNR (that is, excellent electromagnetic conversioncharacteristics can be exhibited) and the occurrence of the head elementchipping in GTT is prevented. It is thought that, in Comparative Example6 and Comparative Example 7, a reason of a decrease in SNR compared tothat in Examples 1 to 9, is due to an increase in distance between thesurface of the magnetic layer and the reproducing element and theoccurrence of spacing loss, due to the coarse surface of the magneticlayer, caused by the oxide abrasive present in the magnetic layer in astate where the FIB abrasive diameter significantly exceeds 0.08 μm.

One aspect of the invention is effective in a technical field of amagnetic recording medium used as a recording medium for archive.

What is claimed is:
 1. A magnetic recording medium comprising: anon-magnetic support; and a magnetic layer including a ferromagneticpowder and a binding agent, wherein the ferromagnetic powder is aferromagnetic hexagonal ferrite powder, the magnetic layer includes anoxide abrasive, an intensity ratio Int(110)/Int(114) of a peak intensityInt(110) of a diffraction peak of a (110) plane with respect to a peakintensity Int(114) of a diffraction peak of a (114) plane of a hexagonalferrite crystal structure obtained by an X-ray diffraction analysis ofthe magnetic layer by using an In-Plane method is 0.5 to 4.0, a verticalsquareness ratio of the magnetic recording medium is 0.65 to 1.00, acoefficient of friction measured regarding a base portion of a surfaceof the magnetic layer is equal to or smaller than 0.30, and an averageparticle diameter of the oxide abrasive obtained from a secondary ionimage obtained by irradiating the surface of the magnetic layer with afocused ion beam is 0.04 μm to 0.08 μm.
 2. The magnetic recording mediumaccording to claim 1, wherein the vertical squareness ratio is 0.65 to0.90.
 3. The magnetic recording medium according to claim 1, wherein thecoefficient of friction measured regarding the base portion of thesurface of the magnetic layer is 0.15 to 0.30; and the coefficient offriction on the base portion is determined using a spherical indentor ata load of 100 micro-Newton and a speed of 1 micron/s on three randomportions of the base portion, calculating the coefficient of friction μfrom the formula μ=F/N, where F is the frictional force in Newtons and Nis the normal force in Newtons, and adopting the arithmetic average ofthe three measured values obtained as the coefficient of frictionmeasured on the base portion.
 4. The magnetic recording medium accordingto claim 1, wherein the oxide abrasive is an alumina powder.
 5. Themagnetic recording medium according to claim 1, further comprising: anon-magnetic layer including a non-magnetic powder and a binding agentbetween the non-magnetic support and the magnetic layer.
 6. The magneticrecording medium according to claim 1, further comprising: a backcoating layer including a non-magnetic powder and a binding agent on asurface of the non-magnetic support opposite to a surface provided withthe magnetic layer.
 7. The magnetic recording medium according to claim1, which is a magnetic tape.
 8. A magnetic recording and reproducingdevice comprising: a magnetic recording medium; and a magnetic head,wherein the magnetic recording medium is a magnetic recording mediumcomprising: a non-magnetic support; and a magnetic layer including aferromagnetic powder and a binding agent, wherein the ferromagneticpowder is a ferromagnetic hexagonal ferrite powder, the magnetic layerincludes an oxide abrasive, an intensity ratio Int(110)/Int(114) of apeak intensity Int(110) of a diffraction peak of a (110) plane withrespect to a peak intensity Int(114) of a diffraction peak of a (114)plane of a hexagonal ferrite crystal structure obtained by an X-raydiffraction analysis of the magnetic layer by using an In-Plane methodis 0.5 to 4.0, a vertical squareness ratio of the magnetic recordingmedium is 0.65 to 1.00, a coefficient of friction measured regarding abase portion of a surface of the magnetic layer is equal to or smallerthan 0.30, and an average particle diameter of the oxide abrasiveobtained from a secondary ion image obtained by irradiating the surfaceof the magnetic layer with a focused ion beam is 0.04 μm to 0.08 μm. 9.The magnetic recording and reproducing device according to claim 8,wherein the magnetic head is a magnetic head including magnetoresistiveelement.
 10. The magnetic recording and reproducing device according toclaim 8, wherein the vertical squareness ratio is 0.65 to 0.90.
 11. Themagnetic recording and reproducing device according to claim 8, whereinthe coefficient of friction measured regarding the base portion of thesurface of the magnetic layer is 0.15 to 0.30; and the coefficient offriction on the base portion is determined using a spherical indentor ata load of 100 micro-Newton and a speed of 1 micron/s on three randomportions of the base portion, calculating the coefficient of friction μfrom the formula μ=F/N, where F is the frictional force in Newtons and Nis the normal force in Newtons, and adopting the arithmetic average ofthe three measured values obtained as the coefficient of frictionmeasured on the base portion.
 12. The magnetic recording and reproducingdevice according to claim 8, wherein the oxide abrasive is an aluminapowder.
 13. The magnetic recording and reproducing device according toclaim 8, wherein the magnetic recording medium comprises a non-magneticlayer including a non-magnetic powder and a binding agent between thenon-magnetic support and the magnetic layer.
 14. The magnetic recordingand reproducing device according to claim 8, wherein the magneticrecording medium comprises a back coating layer including a non-magneticpowder and a binding agent on a surface of the non-magnetic supportopposite to a surface provided with the magnetic layer.
 15. The magneticrecording and reproducing device according to claim 8, wherein themagnetic recording medium is a magnetic tape.